With VAST complaint video ads, you may get a "nested" VAST (VAST Wrappers), which will eventually yield a MediaFile that is a media that you could preload (e.g. MP4/WebM/flv, etc.).
However in the case of VPAID (flash, yuck!), which contains swf as the media files, at the end of the wrappers chain there lies yet another swf - for example, an "ad manager" swf.
So the effect of preloading a VPAID ad is essentially not very useful, because you could not extract the actual media file for preloading merely by inspecting the VAST XMLs. At best - you may preload the final ads manager swf.
The final swf, when played, may then begin to fetch ads from different providers (e.g. brightroll, liverail, etc.), and this process takes a while. At the end of this, an ad might pop up.
Is it allowed to try to preload the actual eventual ad media, or to find out if there is a "fill" (an ad media will be yielded by the ads manager)? For example, by "playing" the ads manager swf in the background, hidden and muted, this might be possible, but it sounds a bit fishy (what if the user eventually does not reach the point where the ad should actually be shown? it would count as fraud).
The answer is No, When you load the VPAID swf --> the ad server count impression and start the bid between the ads providers. You can only load the swf when the user is watching the ad/movie.