I play Salamanders, but without Vulkan. So I think along regular vanilla marine odds...
Actually getting the destroyed result should be your 2nd most common result (by the stats). 1,2, or 3 don't get you much. You can get an "explodes!" on the vehicle damage table with a 5 or 6. The vehicle is wrecked on a 4. It's even better if the vehicle is open topped - it explodes on 4,5, or 6, and is only wrecked on a 3.
I don't think you're going to find much else out there that's going to give you a better chance at getting to the passengers inside. Yes, it's only a 50% probability or so, but then again the whole scout/alpha strike thing is only going to work half the time anyways (if you have first turn). If you're outflanking (thanks to second turn) then the combi-melta still gives you a very nice chance at wrecking vehicles which are no longer sitting still for your powerfist.
The scouts should always get your opponent's attention if you're alpha striking! The key thing is to deploy them so that your opponent can't do anything about it until they've done their damage - then they'll most likely die. So whether or not your sarge carries a combi-melta should be irrelevant, no? If your opponent has a chance to shoot the scouts before they charge (you have second turn, probably) then your scouts should be outflanking.
As you said, it's 10 points and not a big deal either way. I agree the powerfist is more important, if pricier.