Here's some quick math you can double check.
Vex Rune Value:
Worth about 3 Ist runes.
Ist Rune Value:
Worth about 3 Pul runes.
Pul Rune Value:
Worth 3 Lem runes.
Therefore, Vex = 3 Ist = 9 Pul = 27 Lem runes.
Depending on the market, some players value Ist runes even higher, pushing the exchange rate closer to 30–36 Lem runes for a single Vex.
I don't need THAT many, but I am going to need quite a few Lems I'll take obo's until I hit a nice round number for the Vex,
Pm me if you have this sort of quantity of Lem runes
Thanks