So with how OP this is in PvP I was curious to know why it's never mentioned as a viable survivability tool in VMA? Would it take too much of your damage away? Or would it just not proc enough/you die too quickly by one shots? For me, my damage is more than enough it seems but being a stam blade my healing often isn't enough and I die.
As seems to be the case with most other posts on this thread, I haven't tested it personally.
However, given my experience and knowledge of vMA so far, I'm inclined to believe that Malubeth has too low of an up-time to be of any real worth here. I can think of numerous waves (and even specific mobs) in which it could prove useful - the problem is that it is counter-intuitive to how you want to tackle vMA. The ultimate goal is to hit the right target at the right time - and hit them hard and fast.
If your build is one that prioritizes melee over ranged adds, they should be dead by the time they get to you or shortly thereafter. Ranged adds will probably be too far away to proc it realiably, though I can see in certain situations how a stamblade (assuming melee) might be able to proc it before the add dies. Even in that case, however, said add would be dead so quickly that the superior option seems (to me at least) to be something that affords damage. My recommendation instead of Malubeth is 1 Kena + 1 Blood Spawn, which will supplement your damage and healing (both in sustain and healing effectiveness, i.e. vigor).
On a related note: Are you running dual wield? Recent testing has shown Bloodfury and Blood Craze to be a less-than-significant loss in dps for a very significant amount of healing. Remorphing these skills has not resulted in a slower runs or an inability to meet dps checks. From the perspective of group PvE, this still holds true; no significant dps loss in trials or dungeons. I don't have actual numbers to post on this atm, but I can tell you that the dps loss is not a matter of thousands; It's more like a couple hundred at most, over a few minutes.
In vMA, the best defense is a good offense. Kill stuff before they can hurt you. As for defenses, you don't really want to rely on RNG-proc defenses. You should have a defensive measure ready to use when things go south and shouldn't rely on something like Malubeth to save you.