What you read about partial removal is correct.* If the entire malformation was not removed it can grow back.* In fact, it some times can grow back more aggressively.* Please don't take that as a scare...just want you to be totally informed.
A couple of questions:
Did they do any imaging or testing before removal such as an MRI/MRA, ultrasound, angiogram(usually reserved for AVM and VM)?
Was it removed by a surgeon familiar with vascular malformations?
Vascular malformations are not always associated with syndromes.* In fact, the syndromes associated with vascular birthmarks are usually the exception..not the norm.* You will find several people and parents here who deal with these syndromes...as we generally have an overwhelming amount of questions.
If you're at all questioning the results of your surgery, I would definitely seek the advice of a vascular anomalies specialist.* It might not be a bad idea to start right here at the VBF site.* There are several ask the expert doctors available for questions via email.* I've used this myself, and received a reply within a day.* In fact, I'm still communicating with the doctor on a fairly regular basis.
If you have any questions, feel free to email me