>will I be taking on an immature system with problems to replace my mature system with problems?
this made me smile
I guess you will.
But on the other hand, you'd move towards the mainstream where the problems are more likely to be solved,
Seaside in VAST is a bit different, since you need to find ways around using call:/answer: and therefor also WATasks and much of the stuff that you can find in the docs. But once you're comfortable with this, you'll find seaside is quite stable on VAST. It is being used in production already and some projects have come quite close to going live, so you are not on the very bleeding edge if you join in now.
I would suggest playing and learning now with what we have in 8.02 and start seriously when Seaside 3.0 has become "golden" and ported to VAST. It can help to have a Seaside 1-click Pharo Image at hand as a reference sometimes.