Hi again Martha!
Unfortunately, I think that this could be either of those things. I don't have any vascular malformations, but I grew up with horrible leg pains. My middle son has the same problem. Ben, who has several vascular malformations in his legs doesn't ever complain of leg pain. Go figure.
I do have a trick to make it stop. If it is just growing pains, usually a hot bath with stop the pain while the tylenol kicks in. Works instantly for my son (I've run many baths in the middle of the night). Another thing is a heating pad, hot towels from the dryer, and this sounds wierd...but Sam will ask me to sit on his legs while we wait for the medicine to kick in (if we don't have access to a hot bath which is my first choice).
If these trick don't work....maybe you should investigate whether or not it's related to the venous malformation. Or investigate anyway for piece of mind.
(mom to Ben)