Considering Bent has his strings upside down, this is simply a drop D tuning. Still weird though, because VS is very easy to play in standard tuning. Maybe they don't play it in its original key anymore and have gone one step lower due to hard vocal parts… but in that case they could tune all guitars one step down. Oh darn, now I'm puzzled.
Not quite sure why they use that tuning to play it, it can easily be played in D standard. Maybe it's because they have other songs in that tuning (thinking of Serpentine and S.T.G.) and they didn't want a guitar especially for that song, or because the fingering is easier for Bent to use… But there's a really good tab for Vortex Surfer on the site, it's this one.