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).