my vox wah kills my tone by making it sound weaker overall. I mostly play plugged into guitar rig now with my wah leading into my interface, but it does it in my amp too. Is this normal? Are there any 'non-tone killing' wah pedals out there?
Do you run it on batteries or power supply?

Could be that the battery is dying
My Rig:
Maverick F-1, Ibanez RG1527, Schecter Omen 8
Marshall JVM 410H,
Hand built 4x12 w/ V30s
Current board:
MXR M132
MXR M101
MXR M108
I use a VOX wah and unless you have a good power supply it does sound weak. If you are using batteries this could be your problem. I also have an Ibanez Weeping Demon and it's even more obvious with that wah. They eat batteries quickly. If I play one four hour gig using a new quality battery, I will notice a drop in the wah's output by the last set.
You might need to convert it to true bypass. Do you notice any tone-sucking when the pedal is turned off?
not when its off. when i turn it on its an instant thinning out of tone.
Could just be a case of too much treble, try opening up the Wah and adjusting the gear/tooth position of the pot.
If you going to open the wah and change the gear/tooth setting, get a marker and mark the place where it is before moving it. I have done this without making a mark and it's a bitch trying to find the original location if you want to put it back the way it was.