you'll get 10 different answers
I won't attempt to answer the question since we don't have nearly enough information to validate nor rebut any of the other suggestions, but maybe we can give you a little more insight as I let the first random thoughts that enter my mind flow into this post.
How many site A employees will be working at site B? They will remain site A employees while working at site B? Will they be accessing site B IT resources while working at site B? Are they permanent transfers or rotating crews?
Immediately I see at site B two potential problems inherent in SBS. Problem 1 is a hard licensing limit of 75 users; not sure if the people transferring from site A to B will flirt with that limit (if they'll even require access to site B resources).
The second, and biggest problem with SBS relative to your situation is that SBS will not allow any TRUST relationships. If site B was on W2K3 Server non-SBS, you could set up a TRUST with the 2 domains and call it a day, however this is not the case with SBS in the picture.
I'll need to make one comment relative to directors not wanting to provide funding. They've allocated funding for many different parts of this project, whether this is a merger, buyout, or just company "right-sizing," or whatever this deal is. It is unreasonable to expect that with such a transition, the IT infrastructure will not require funding for any changes, and it was an oversight on their part if they actually believed that.
OK enough lecturing from me.
A. Implementing VLAN's does nothing for you by itself; the actual solution of this recommendation is transitioning both IT infrastructures into a forest with the two child domains. This is probably the most robust solution, especially if there's a chance that the sites may do more together in the future, though it could be overkill if the sites will not be.
B. A gentler approach to solution A but along similar lines, may be transitioning site B's infrastructure from SBS to the full-blown components so that a trust relationship can be put into place between the 2 existing active directories. Doing this, site A employees would be able to log onto and access site A resources from site B workstations.
C. Another option may be to set up Term server(s) at site A such that site A employees at site B, can Term Service or RDP into site A to access site A resources as needed.
D. With a 100mb pipe, you can even install site A dedicated workstations at site B.
Hope this is helpful
