Andy_L:
One factor is that I think Vanko was surprised that Tony was in the car - he expected Tony to be in the stands watching the race, and hoped to humiliate Tony by destroying the Stark-branded car with a super-suit, just after Tony had publicly stated that no one could produce such a suit but Tony (and further expected that Tony would stay in the stands, thus appearing as a coward). Having Tony turn up right in front of him was unexpected, and it took Vanko a moment to reconsider his plans.
It also does no good for him to beat an un-armored Tony Stark - thats easy… beating IronMan - thats something.
Trying to explain/justify comic book physics, events, and psyches is simply a waste of time. Things happen the way they do because they need to for the particular story to proceed as the writer desires. Suspend your disbelief.
They’re making that as difficult as they can.
DigitalC:
Or, more likely, based on his father’s technology.