Remember how to test for a vacuum leak:

1) Start engine
2) Drive car and warm engine up
3) Park car, do not shut off engine
4) Raise hood (I suggest using the release lever under the dash)
5) Observe the engine's idle speed and it's "smoothness" or lack thereof in idling.
6) Take oil filler cap off.

IF: the engine starts to run rougher than in 5) above, or if it seems to struggle a bit to maintain idle - that would be a sign that you have no vaccum leak because removing the cap creates a HUGE leak.
IF: the engine's idle does not change over what you saw in step 5) above, then you must have a pretty good leak if creating the huge oil-cap-off-leak does not make a noticeable difference!