The soffit vent plates are, basically, screened vents on the underside of the eaves. The best-case scenario for ventilating the roof would be to have a plate installed between every rafter. Unfortunately, all too often, I don't see that. My home is vinyl-sided too. When I installed the soffit material, I actually ripped out the the closed soffit that as installed with the home. In doing this, I completely exposed the eaves and then installed VINYL soffit material. Hence, I accomplished the scenario of having EACH and EVERY rafter 'shoot' vented. So, to answer your question(s), yes, it's possible you don't have enough under-eaves vents to cool the roof-line. And yes, air should find it's way into the attic BUT, if there isn't enough venting, the attic will stay relatively hot.
If you can get up into the attic, you can see if there is any insulation or whatever 'blocking' any of the vents. Also, DON'T stack boxes and stuff up into the cavity between each rafter. If you do this, you'll inhibit the flow of air along the roof-line. Most commonly, the stacking of boxes in the attic along with 'blown attic insulation' covering the vents is the common cause for VENTED attics staying hot. If you need more info, post up. My best to ya and hope this helps.