While Vietnamese military chiefs couch the build-up in purely defensive terms that reflect a growing economy, People's Liberation Army strategists confirm privately they are watching developments.

'Of course, such developments are natural to a certain extent,' one PLA strategist said. 'Our main concern in the short-term is that Hanoi becomes overconfident and all these plans lead to bellicosity. They must know that would be destabilising.'

A key part of the evolving naval strategy is Hanoi's broadening and deepening military relationships. While Russia - Vietnam's cold war patron - continues to be vital, recent acquisitions have included deals involving the Czech Republic, Canada, Israel and, significantly, India.

The Indian-Vietnamese military relationship is being closely watched in Beijing, where officials have formally and repeatedly objected to Indian oil exploration deals in disputed waters off the Vietnamese coast. Those objections, however, have not stopped a military relationship from expanding on several fronts.

Not only are the two sides negotiating for Vietnam to purchase its BrahMos supersonic cruise missile, but also further submariner and pilot training and upgrading port facilities in the south-central port of Nha Trang. The provision of used Indian patrol craft is also under discussion, according to diplomats and Indian press reports.

Indian naval officials are now frequent visitors to Vietnamese naval facilities.

'It makes sense of on several fronts,' said one veteran Asian military attaches monitoring developments. 'The Indians can help Vietnam perfect using all the Russian kit they are now acquiring. And it allows Hanoi to not just sustain but deepen ties with New Delhi and Moscow, both of which offer a kind of leverage against Beijing.'

Regional diplomats are also watching the emerging US-Vietnamese relationship. The two former enemies are currently negotiating a formal 'strategic partnership'.

Vietnam is hoping to overturn long-standing US congressional opposition to arms sales, with an initial wish list that includes spare parts for former US Huey helicopters and advanced coastal radars. The US is also supplying Indonesia and the Philippines with similar coverage.

Other developments include:

The delivery from Russia late last year of two Gepard class guided-missile frigates equipped with anti-ship missiles with a range of 130 kilometres and two Svetlyak class missile patrol boats. The first four of 20 more Su-30 jet fighters with advanced anti-ship cruise missiles also arrived. The Russians have sent a second Bastion ballistic missile system to buttress Vietnam's coastal defences.

Approval from Prime Minister Nguyen Tan Dung during a mission to the Netherlands to buy four Sigma class corvettes.

The emergence of a domestic military shipbuilding operation. A fast patrol boat and a troop carrier were completed by a shipyard in Haiphong in September and October - efforts that are expected to be followed by the construction of Sigma and Molniya-class vessels with Russian assistance.

Significant increases in naval budgets and salaries. Defence intelligence group Jane's recently estimated that Vietnam's naval procurement had soared 150 per cent since 2008 to top US$276 million last year. It is due to hit US$400 by 2015, when the first Kilos are due to be in service.

Professor Carl Thayer, a veteran scholar at the Australian Defence Force Academy who has logged the recent acquisitions, said Vietnam was also creating a joint naval and air command to cover its increasingly important maritime domain, including its 3,200-kilometre coastline.

'We are talking about very significant developments here, but they are going to have a great deal of difficulty in pulling all the bits and pieces together into a workable whole,' Thayer said.

'Vietnam knows it is never going to be able to compete head-on with China's military build-up, but what we are seeing is the creation of a credible deterrent long-term.'

London-based independent defence analyst Trevor Hollingsbee also said that, despite expanding acquisitions, Vietnam would face a major logistical and organisational challenge in tying its assets together.

'That said, they have proved themselves repeatedly to be a highly efficient military and must not be underestimated,' he said. 'They will certainly be giving the Chinese food for thought.'

One of Vietnam's biggest challenges long-term will be the defence of the bases and garrisons it keeps on 24 islands and reefs in the Spratly Islands - significantly more than other claimants. It is an occupation that stirs considerable angst among Chinese nationalists online.

China and Vietnam last fought over the islands in 1988, when PLA naval forces took six reefs from the Vietnamese - an act not forgotten in Hanoi.

'Those islands are something of a double-edged sword for Vietnam,' Hollingsbee said. 'They are difficult to defend, but they would also be difficult to take and sustain for any invading force.'

Gary Li, a PLA analyst at the private intelligence firm Exclusive Analysis, said some of the equipment obtained by Vietnam would represent only a theoretical deterrence.

'A new weapon like the BrahMos cruise missile is still untested against an actual enemy,' Li said. 'It is very hard to tell the precise threat it is against the latest Chinese ships and their countermeasures.'

Out with the old ...

Shershen-class torpedo boat

16 bought from Russia between 1973 and 1980 - only four left in service.

Petya-class frigate

Six 1950s anti-submarine ships from Russia. Only Vietnam and Syria still have them in service.

Svetlyak-class fast attack craft

Used for: maritime border patrol, protection of other vessels

Weapons: guns and torpedoes

Range: 3,500km

Top speed: 30 knots

Origin: Russia

... and in with the new

BPS 500 missile boat

Used for: anti-submarine defence

Weapons: missiles, guns and torpedoes

Range: 3,000km

Top speed: 32 knots

Origin: Vietnam/Russia

Gepard-class guided missile frigate

Used for: escorting friendly vessels, area patrol

Weapons: anti-ship missiles and guns

Range: 8,000km

Top speed: 28 knots

Origin: Russia/Vietnam

BrahMos supersonic cruise missile

Used for: primarily attacking sea targets

Speed: Mach 3 (said to be world's fastest)

Range: 290km

Origin: Russia-India joint venture

Bastion anti-ship missile system

Used for: protection of coastline up to 600km long

Range: 300km

Origin: Russia

Source: International Institute of Defence Studies, Tokyo