I too am also considering getting VSL to run under Halion. To be honest, I don't really care what sampler I use, since all I am really interested in is using a VSTi instrument and am not interested in programming it at all. Ideally, the Performance Tool and the samples would run under a VSIi so I didn't have to be bothered with routing out to Performance Tool. However, I imagine this will come along eventually and can easily live with it meanwhile. My idea is to have virtual orchestral instrument players that give me a nice abstract interface where I just tell them the style to play and off they go... longer term, I want to see us moving away from particular libraries and bundling library with sampler players, much like QL does.
I too am an ex-Giga user, who gave up using Giga several years ago, having since ported my garritan, dan dean et al libraries to Halion with virtually no problems at all now, being able to run them in a lovely VST environment and with more reliability and polyphony that Giga.
I don't like the idea of having to convert from giga to halion, especially since I am planning on shelling out 3k+ for the pro edition, but I will do, unless Tascam can prove me wrong with Giga3, if it comes out quickly and if it is fully VST compatible AND is more efficient. But, to be honest, any company which disappears for two years without saying a thing about what it is doing and without posting any updates to their current software... well, I don't have a lot of faith in them. I'm willing to be proved otherwise.
Any tool that is not VSTi these days, I am not interested. That is just me and the way I work.
So, in conclusions, VSL, listen to your potential Halion customers... not everyone wants to stick with giga... you have a load of customers using Halion.