well if you want a vita you can get one if you want e3 is only a few weeks away you could wait if you want
Ask yourself this question: Are there any games I want? If you answer yes, get a Vita now. If you answer no, wait till E3.
if you want to buy it brand new, it doesn't really hurt to wait until e3. if you want to buy a used one on ebay however, it's always possible that e3 spark some interest back into the system and see price climbing.
I'm waiting for E3 for a price drop and hopefully some new games. Right now all I want to play are some old PS1 games, which I can do on my PSP 1001.