First off I like the idea of the deck you've got, it should be nice and strong in an aggressive play style.
My suggestions for changes are to take out:
2
Falkenrath Gorger - I don't think it's necessary as most of your vamps already have madness
2
Voldaren Duelist - turn 4 you're not likely to be facing one creature that you don't want to be able to block, the haste is nice but a four drop 3/2 is not great value IMO
2
Stitched Mangler - again I see the appeal, you're tapping a creature and keeping it tapped, but for one of those turns it itself is going to be tapped and unable to do anything so again a three drop 2/3 worth it for this deck.
1
Compelling Deterrence - even if you use this on yourself as a way of saving something and giving a madness trigger I'd be tempted to take it out just because as a 1 off in a deck a. you won't find it when you need it and b. when you do find it you won't want it (sod's law)
2
Rush of Adrenaline - IMO this deck doesn't need this trick, I'd rather see an madness trigger into another vampire out rather than save mana for a trick.
1
Malevolent Whispers - too expensive for the effects, I like the thinking but are you ever going to be thinking turn 4, "gosh I wish I could grab their creature for just this turn"? If you had sacrifice outlets i'd be behind you, but as it is I'd drop it.
2
Welcome to the Fold - again this seems great until you analyse when you'll play it, turn 3+ you can nab a creature,
but you have to hold mana up for it and it has to be a lot of mana if you want a big creature.
3 Avacyn's Judgement - again this is a good card but like
Welcome to the Fold it's an all in card when you trigger madness you either pay all your mana for a big burn or you might as well not bother and personally I'd rather build board presence.
I know you said you'd like to stay SOI only but another swap I'd consider seriously is to take out:
2
Nagging Thoughts2
Pore over the Pagesand put in:
2
Artificer's Epiphany - cheaper than
Pore over the Pages (even with the land untap) and better than
Nagging Thoughts since it also gives a madness trigger.
As for fancy lands you're sort of stuck R/U in Standard, the ones that come to mind are
Shivan Reef (Origins) and
Wandering Fumarole (OGW), you could always stick in a
Westvale Abbey if you have one.
Just my thoughts.
Rich