The most often used method to play multiple VAST ads at different time positions is to use VMAP protocol. VMAP is used by many ad providers, including Google. VAST plugin by Nuevodevel.com includes experimental support for VMAP. Example test of VMAP Ads Playlist is available to review as well.
An alternative for VMAP protocol is Ads Scheduler implemented in VAST/VPAID plugin, which also allows to define and play multiple VAST ads.In example above you can see total 5 VAST ads. 2 x preroll, 2 x midroll at 00:15 and 1 x postroll ad. Below you you have example of ads setup code.
player.vastAds ([ {tagURL: "preroll-1-ad-tag-url", id: 'x01'}, {tagURL: "preroll-2-ad-tag-url", id: 'x02'}, {tagURL: "midroll-1-ad-tag-url", id: 'x03', timeOffset:"00:15"}, {tagURL: "midroll-2-ad-tag-url", id: 'x04', timeOffset:'00:15'}, {tagURL: "postrol-ad-tag-url", id: 'x04', timeOffset:'end'} ]);