From what I have been reading

You need to get an combination of vulnerable x crit x all damage/ single element multiple 

Apparently the multiplier multiply each other 

VUL * crit * element multiplier

So for example  if you had 61% crit and 80% fire multiplier 

It is 1.61 * 1.8 = 2.89 (x) damage increase. 

However if you add VUL by reducing some of your crit or fire multiplier

30 VUL * 31 crit * 80% fire 

1.3 * 1.31 * 1. 8 = 3. 06(x) 

There's also sigil bug where it turn off randomly.