This is the last name of a French mathematician. I would like to know how it is pronounced.
Is it a common last name?
My maths teachers and fellow students pronounce it in the expected way:
/vɑ̃.dɛʁ.mɔ̃d/ (in IPA).
This being said, this name is not of French origin, so this pronunciation may not be etymologically correct.
It is not a common last name. For instance, geopatronyme.com references only two births between 1891 and 1990. (I make no guarantees about the reliability of this site.)
I'm from Belgium and it is a typical Dutch name (Netherlands or Belgium). His name's history indicates the place you originate from, like you have de in French.
About the pronunciation, you can't make a big mistake. You will hear a slight difference when hearing it from English, French or Dutch speaking people (French speaking people won't pronounce the ending “e” as much as for example Dutch speaking people).
But overall, as long as you put the emphasis on the “mon”, you should do fine.