OK... Bali seems to win...!
If nobody objects, I'll just flesh out my reasons for suggesting Saigon....
-good place for me to go alone (i.e. not looking for some honeymoon place)
Its a buzzing city... You can wander around markets, enjoy the night life, no need to be bored. English is well spoken where/when it matters. Vietnamese people are a friendly bunch. If you return to the same coffee place more than a couple of times you begin to feel you're not alone. Also there are some interesting museums/exhibitions you can go to. This is especially so if you're interested in some of the political history, kitsch propaganda art etc. There are "expat" places where you can have intelligent conversation with foreigners who live/work there and might show you around a bit. Or if you prefer you can hang around the backpacker/tourist places and compete on the basis of how dog-eared your LP guide is, how little you paid for your cyclo ride, and what's the cheapest way of getting to cambodia. My point is tourism is not the primary reason for its existance - there's more to it than that. And just by being there you won't necessarily be taken for a tourist.
-relaxation is important, but i'm not a complete vegetable
I'm a lazy sod, and being a warm place I'm not keen to rush around. If you can ride a bike, you can hire one and shoot around like the locals do. There's the usual stuff that most cities would have - bowling alleys, fitness centres etc. I tend to hang around coffee shops. If I wasn't so ignorant, I'd probably read books there. What a pity I can't show off to my friends about how I read "The Quiet American" whilst at the Continental Hotel.
-i like beaches and SCUBA, yet i'm not completely adverse to the inland stuff
Saigon falls down here. For beaches you could go to Nha Trang, Mui Ne, or Phu Quoc. All require a journey of some sort - you need to spend at least a night away for it to make sense. Trips up the Mekong are always well spoken of. But the city grips me more than would a muddy old river or....
-i need to be able to get myself to sin without breaking bank
You can get yourself to sin or into sin pretty cheaply.
-speaking of bank, i'd prefer to keep my land budget under $500 (for 4-5 days)
No problem. Not sure what diving costs though. These people are the most well known:
http://www.divevietnam.com/
Saigon is changing rapidly as its economy booms. Trendy "coffee shops", night clubs etc popping up every 5 mins. I feel this is a further reason to go, it could be different and more expensive in 5 years time and anyway its nice to be in a successful place. Even the old terminal at Saigon's Tan Son Nhat airport will be gone shortly.
Everbody goes to Bali - probably why I've never been there - perhaps if I had I'd be extolling it too!