I've heard of the troubles with OSX... Seems like Apple jumped a bit too far ahead on that one, and left a fair few people behind. All the advice I heard was 'Stick with 9 for now and swap to 10 when all the problems are ironed out.'
Not much help here...
Contact the suppliers of the soft synths you wish to use, and see if they produce plugins that are compatible. If they don't, ask them when the products are likely to appear. All the major companies will cater for OSX, but there may be a wait while they all catch up.
Also, I *think* I've read about a 'wrapper' program which will allow you to use all those VSTs in Logic on OSX. I'll see if I can hunt down the article tonight and find out if it's appropriate.
Is there the same problem with Cubase on OSX?
T*