sorry mate, aint good enough

1. Get Ancient Path Checkpoint
2. Run through East/West channel
3. Run through Dahlgur Oasis (do all extra dungeon)
4. After clearing Dahlgur's enters Ruined Cistern (next to Path to Oasis's Waypoint)
5. Tele back to town, go through Sewers of Caldeum.
6. Rinse and repeat.

You CAN get 5 stack of NV in Vault of the Assassin.. but it also drops another legendary ring; Manald Heal. It would be pretty discouraging and demotivating if you found a legendary ring, but not Signet. You CAN NEVER get Signet in here.

Those invi-snake has good chances of dropping them. As they're in level range with the Signet.
Forgotten Ruins in Dahlgur Oasis has a gooding droprate. I've gotten 2 Signets there.
Sewer of Caldeum also

Some other tips.. Loot some rare ring also. I've pickup few good rolled Rare Ring (80Str, 16 Exp, 4-8 damage, Socket) sold for 20m

It's all about RNG

^Monster level too high to drop Signet