1. Why are you using a 64-bit OS for Mame? Are you seriously expecting a performance boost?
2. Why are you using Vista? If a performance boost is what you are looking for (I assume from question #1), then you do NOT want to be using Vista...

Not to put words in Lanman's mouth but:
1) Because he wants the average 20%+ preformance gain that comes form running MAME 64bit on a 64bit OS.
Aaron ,
John IV,
myself and a few others have all post benchmarks that show MAME is noticable faster in 64bit. Anyone with a Athlon64 X2 or Core 2 Duo should expect to see a preformance jump when switching to a 64bit OS.
2) Why Vista you ask: (I'll quote myself on this one to save time)
- It cheaper: you can buy Vista Home Premium 64bit cheaper than you can Windows XP Pro 64bit. (Of course there are pirate copies of both available)
- It easier to get: alot of places will look at you blankly if you ask for XP Pro 64bit, Vista 64bit is easily found in almost all computer outlets.
- Drivers: this is the big one, Vista 64bit has good driver support for most system harware, and average driver support for periphals. both will only improve with time, XP 64bit on the other hand was basically still born and as such has only average driver support for system harware, and rubbish support for periphals.
- General Support: Vista is new and while most people don't like it support for it will only increase over time, XP 64bit is very little support and that will only get worse with time.
The only problem I've seen to date with running Vista 64bit in a MAME cabinet is avialablity of lightgun drivers, but this is more of a 64bit OS problem than just a Vista problem.
The main complains about Vista are:
- The endless warning messages, easy fixed turn UAC off.
- It's slow, again easy fixed, only install it on a system with 2GB of RAM and either a Athlon64 X2 or Core 2 Duo. (Find it's very quick actually)
- It bloated, (can't count the number of times I've heard that about XP over the years),Simple, strip it to the bone with vLite.
- Incompatiblity, who cares MAME 64bit works purfectly, most of incompatible software and hardware is irrelavant to MAME users anyway.
Simple fact is most of the people who complaining about Vista would be lucky if they've played with it for more than 5 mins in a department store.
He didn't say he wanted to take it to the next BETTER level. 
Seriously, that kind of stuff won't help MAME. Get a copy of XP and strip as many of the unnecessary Services out of it that you can, either manually or using a gaming app.
What can i say to that other than your wrong, MAME 64 running on Vista 64bit will always beat MAME 32 on even most optimised install of XP.
It really shocks me that two members of this forum who have posted here more often than me by a factor twenty each, have not managed to pick up on the fact the MAME 64 on a 64bit OS is faster than MAME 32. Hell my benchmark thread alone is fast appoaching the 10,000 views mark, I'm surprised with those kind of numbers that everyone on this board hasn't read it atleast twice by now.