vulnerability to all and multi-type damage

[kuwojin]

our cleric give vulnerability 10 to all damage to one enemy，then one ally attack it with fire and lighting damage，the enemy take 10 extra damage or 20？

 

[AbdulAlhazred]

10. I don't know any wording that would increase the damage multiple times. TBH though, the RC's text on the subject actually manages to avoid clarifying this point. Multiple resistance/vulnerability are NOT cumulative, but that is only talking about multiple instances of the SAME type not stacking. It never outright says that you wouldn't apply 2 different vulnerabilities at the same time, though it is made clear that vulnerability to ANY damage type which the damage has is sufficient to trigger the extra damage. How you extrapolate that to your situation is really up to the GM.

Basically, I figure if the PCs can engineer it so they create this situation (by causing a creature to have an added vulnerability or exploiting an existing one by adding damage types) then why not give them the fruits of their ingenuity? It isn't likely to come up super often.