Is there any difference in completing two 250 points vows over one 500 point vow though? Besides of course taking up two of your maximum total of five vows.
No there isn't. The only reason that there is a limit on vows is to avoid the craziness of last minute vowing of mostly single mini units creating a bottleneck in the closing moments of the event. Also it helps people to organise their project(s) think ahead and think strategically, especially when combined with the points per vow limitation.
Anyways, there is no prize for the number of vows - they are only depicted in the Custos Fidei to commemorate its participant's efforts.