Adequate, certainly - the deer paddocks were OK, for example - but none of the exhibits went beyond that, I'd say. The general level of care was not disastrous by any means (enclosures were clean and animals well-fed and for the most part healthy-looking), but the exhibits were very much basic and often too small for my liking. And it's really not big on aesthetics - the most attractive exhibits being either the elephants or the ostriches - neither of which are exactly stunners!
It's a bit of an odd situation - although Hanoi is the capital, Saigon/Ho Chi Minh City is where all the money is. We were only in Saigon a day (and spent most of that in the zoo!) but it felt very different. In many ways much less interesting for a tourist, but somehow more 'comfortable', more neat-and-tidy and tangibly wealthier.
I think if you visited both without knowing you'd be very surprised to learn Hanoi was the capital and not Saigon (although of course there's a very good reason it is not the other way round).