By Vantage do you mean the DB7 V12 Vantage? If so, the DB9 is an update of the DB7 based Vantage. The DB7 Vantage is no longer in production, so if you are considering one it would be used (sorry, "pre-owned"). DB9's are just now becoming available (at least in the States), and you may have to get on a waiting list. As to how they compare...sorry, but I have no practical experience. The DB9 is just too new and I don't even know if the automotive press have had a chance to sample them. I don't believe I've seen any articles or road tests on them yet.
Having said all that, they should be similar or comparable cars. They are meant to fill the same niche in Aston's lineup. The DB9 is being pitched as a middle-market Aston, between the new "entry level" V8 and the top-of-the-line Vanquish (which is itself at end-of-life). Aston has launched their LeMans bid (through Prodrive), so they are definitely harking back to their heritage and making no bones about their sporting pretensions. All of these can only be good things.
Let us all know what you decide, and by all means go for it - I'd think you could never get a truly "bad" Aston (although some owners of older models might cringe when they read that and then look at their checkbook balances after a restoration!).
Good luck.