I wouldn't say vtec's are "rare"... but thery're definetly harder to find than a civic vtec, or the like.
Most people that have a vtec prelude have swapped the H22 into their Si or S model. Sounds like you got your hands on a nice ride, especially with under 91k. Some things to keep in mind though:
1. Buy a HELMS manual.
2. Ask original owner if timing belt has been changed... with 91k its about due.
3.
www.hondaautomotiveparts.com is your best friend
4. Don't be afraid to ask questions
5. Don't be around to SEARCH first :P
6. Please don't rice out your are :P Prelude's are too nice for that mess.
7. Enjoy your stay!