Okay so one cannot see it on other players, but isn't the buff shown on yourself if you get it from another player?
You coudl test this by enabling vanilla UI buffs on yourself in the settings and test it with a friend in a duell.
So at least your own vulnerability should be trackable via Srendar or other addons then.
And this should be trackable via enabling the debug info in Srendar and check the [123456] id on your buffs.
UESP data about it:
http://esoitem.uesp.net/viewlog.php?...kills&id=86284
Data about skill ids/buff ids for the search term "onslaught". Maybe the name of the buff is different though?!
http://esoitem.uesp.net/viewlog.php?...pe=minedSkills
83229 minedSkills Onslaught Strike at an enemy with a vicious blow, dealing |cffffff1478|r Physical Damage to them and all nearby enemies. This attack ignores the target's Resistance and grants you Physical and Spell Penetration for your Direct Damage attacks equal to the amount ignored from the initial target for |cffffff9|r seconds.
86278 minedSkills Onslaught Strike at an enemy with a vicious blow, dealing |cffffff1478|r Physical Damage to them and all nearby enemies. This attack ignores the target's Resistance and grants you Physical and Spell Penetration for your Direct Damage attacks equal to the amount ignored from the initial target for |cffffff10|r seconds.
86281 minedSkills Onslaught Strike at an enemy with a vicious blow, dealing |cffffff1478|r Physical Damage to them and all nearby enemies. This attack ignores the target's Resistance and grants you Physical and Spell Penetration for your Direct Damage attacks equal to the amount ignored from the initial target for |cffffff11|r seconds.
86284 minedSkills Onslaught Strike at an enemy with a vicious blow, dealing |cffffff1478|r Physical Damage to them and all nearby enemies. This attack ignores the target's Resistance and grants you Physical and Spell Penetration for your Direct Damage attacks equal to the amount ignored from the initial target for |cffffff12|r seconds.
83230 minedSkills Onslaught
83231 minedSkills Onslaught
83345 minedSkills Onslaught Passive
83346 minedSkills Onslaught
86279 minedSkills Onslaught
86280 minedSkills Onslaught
86282 minedSkills Onslaught
86283 minedSkills Onslaught
86285 minedSkills Onslaught
86286 minedSkills Onslaught
95263 minedSkills Onslaught
95353 minedSkills Onslaught
126497 minedSkills Onslaught
Maybe one of the 83 or 86, 95 or 126 ids is the buff you are looking for.