Great question -
You raise many issues -
The first is the concept of "VBAC" or vaginal birth after C-section. While the debate has gone back and forth most obstetricians and nurse midwives would agree that patients who have delivered by c-section previously are much more likely to have a c-section again.
The reason this should be a consideration in your planning is that you correctly surmise that some distortion, change in shape or trauma to the labia can happen during childbirth.
However, it is important to recognize that over 75% of women getting a labiaplasty have it before they are done having children.
The decision is yours and your doctor's - you need to assess your concerns and the benefit you may have by pursuing the procedure now vs. the risk that you may need additional surgery after additional children (most practioners would ascribe a small risk to that possibility.)
I hope this helps.