There are several antivirus programs currently fully compatible with Windows Vista, including, but not limited to, Avast, AVG, TrendMicro's PC-Cillin, and eTrust EZ Antivirus. (Those four are all free or free-to-try.) Special corporate editions have also been available from Norton and a few others since early in the beta process.
Now, while I cannot comment on exactly how many flaws have been found, it's nothing abnormal. Software in its early stages is expected to have flaws, and the whole purpose of the beta process is to find as many of those flaws as possible before it is released. Those who decide to beta test know the risks going in and are willing to accept them in order to experience the new product and/or to help find bugs in the software. It's just part of the territory.
Microsoft has identified over 5,000 software and security flaws already! What's even worse is that there isn't an anti-virus program out there that's compatible yet! DON'T SWITCH UNTIL THE PROBLEMS HAVE BEEN FIXED!!!