Have you tried looking at guitar tablature sites? Most times they tell you what the tuning of the song is. And open D tuning is when you tune the low E string to D tuning. I use that tuning on my bass a lot and it's a great sound.
Have you tried using a capo.....im just learning to play myself but have noticed that most blues musicians play with a capo live. Also it si possible that he used a couple of pedals....although a doubt a virtuoso like him needed them for most things.
srv did tune down one half step below standard tuning;also he did use 012-058 gauge strings and gibson jumbo frets;for tabs you can go to any tab site or follow the link on the harmonycentral.com.I hope this helps.