SVOBoy: It looks to me like all Honda did was to build the head without the additional valve's in it. I can't see any reason why the 8 valve cam wouldn't work just fine on the 16 valve head.
I think the primary reason that the HX only put's out 70HP is probably, largely because of not having the second valve in the head. From what I've read it sounds live the VX just resolves that issue by allowing the second valve's to come on above 2000 rpm.
One possible issue: On the information I've read on the VX head, they keep the second valve's open a miniscule amount to keep fuel from collecting in the top of the second valve. I don't think this would be an issue because you never would be opening the second valve's.
SVOBoy: I don't know if you would encounter any problems, but doing the cam swap is pretty easy, for someone with your skill's. Basically you have the following step's.
-loosen timing belt and slide off of cam.
-remove cam bearing cap bolt's
-pull camshaft out.
-reverse the whole process with the exchange.
-reinstall timing belt
-reset timing on distributor
I don't think it is a real big deal, to try it. Additionally, all you lose is the time to change it back. After seeing your picture's, I think that your biggest problem will be that you will be at the 70HP, because of your reduced valve capacity, which you would have anyway, if you do the whole head. I'd certainly give the 8 valve cam a try, just because it's not to big a deal, you can't hurt anything and you will most likely get the same results as if you had swapped the whole head assembly.
Bunger: You referenced longer runners for the VX head. So, what does that mean?
The runners are different in the head, not sure if they are longer (or shorter). If you've ever looked down the intake ports on a 16V head, you can see how the passage way splits into 2, each going to two different valves. By just having one valve not open, you're not going to get very optimal air flow. The same goes for the exhaust side.