First of all, make sure that you have good tuners. It may seem obvious, but I have a Martin 12-35 that was a bitch to tune until I replaced the stock tuning machines with the recent Stew-Mac offerings.
The next question is whether you're talking about retuning when you are performing or playing at home, etc. If you're performing, it might be best to use Spanish for your 12 string tunes in one set and Vestapol for the 12 string tunes in the other. (Retuning in between sets.) Unless you have retuning down to a fine art, your audience might run short on patience. Of course, you could always pass it off as some Avant-garde experiential sounds thrown in for their musical enlightenment, but I wouldn't press my luck.
Edited to add: If forced to choose a sequence, it would be: Standard, dropped D, Spanish, and then Vestapol. As far as I can tell, this minimizes the number of strings that need to be re-tuned between tunings. I also find that using a flat pick helps me when tuning / retuning. If you're playing out, you're going to have to mix keys so your audience's ears don't doze off. If you only have one guitar and it's a 12, this naturally presents a challenge.
