WE'VE decided to change our career and become an American banker - with a 12-month or more bonus, luxury company flat, eight weeks holiday a year . . . Wait a moment, you must be thinking, that last bit doesn't sound right. But a reader has just called in with a sneak preview of the vacation-filled year ahead for the Bank of America's international private banking division. A 1994 desk diary from that division had been received with suitable gratitude - until she tried to write in an appointment for April 13. It did not exist. Indeed, our forward-thinking reader said, there were no pages for eight weeks between Wednesday, April 6, and Thursday, June 2. This was ''excellent news'' for Bank of America employees but ''not such good news for Tauruses and some Geminis, who miss their birthdays altogether this year''.