I've read in several places that this is actually a good tanking glyph. My gut feeling has been that I wouldn't use it even if it didn't occupy a glyph slot but I haven't actually looked at what it does.
Unglyphed Vampiric Blood increases healing taken by 25% and gives a last stand ability for 15% of my maximum health. These effects last for 10 seconds and the ability has a 60 second cooldown. Typically I use it if I unexpectedly dip low. Frequently I also hit Rune Tap at the same time. Sometimes I just use it as a 'tanking cooldown' on high damage aspects of a fight when Icebound Fortitude is on cooldown.
Glyphed Vampiric Blood increases healing taken by 40% and has no other effect. It lasts the same amount of time and has the same cooldown.
So, what does that actually do? If I chain it with Rune Tap then unglyphed I immediately get .15 of my max health plus 1.25*1.15*.1 of my max health for an immediate heal of .29375 of my max health. If I chain it with Rune Tap glyphed then I get 1.4*.1 of my max health for an immediate heal of .14 of my max health. So right off the top I'm worse off, but eventually I'll be better off. After X raw healing with Y raw max health I'll be at either 1.25*X+.29375*Y or 1.4*X+.14*Y. The equilibrium point is when X = 1.025*Y. Or, after I've been more than healed to full. So after I've been healed to full any extra healing comes at a discount for the healers (or if it's steady huge damage the extra healing percentage could theoretically keep me standing).
Without rune tap the math is even simpler. 1.25*X + .15*Y compared to 1.4*X. This reaches equilibrium when I've been healed for my max health.
No matter why I'm hitting the button, if I end up at max health I'm going to be satisfied. Unglyphed is strictly superior until I get that much healing poured into me. Glyphed could only makes sense if I never get topped up to full but need healing for way more than my max health in 10 seconds and a 25% boost to healing isn't good enough. (Or this happens every minute and the healers barely run out of mana?) I just can't see it ever being useful.
I feel vindicated.