Can sperm travel through underwear and onto the vaginal lips? if so, how likely/unlikely is pregnancy?

Not likely: Not likely. Do a pregnancy test around time of next period. I wouldn't worry about it otherwise.

Still risky: Barrier methods to avoid conception in the form of condoms remains in the 80% range as effective contraception. As a result, undergarment are less of a barrier since they have the ability for fluid to leak through therefore it is not an effective method to avoid pregnancy.