Honestly, it depends on how you plan to play. If you group up or do a lot of ranked, get full tank gear with the 2 vindicator armorings. This is because in group play your job is to guard, disrupt, taunt, and CC and depend on your group for dps.
However, if you solo queue regs a lot, you're better off speccing full Vengeance and switch to tank stance if you want to guard. It's not as tanky as full tank or hybrid, but it does the job well enough in regs while maintaining a lot of damage. If you're careful with focus management you can still pull 1k+ dps in tank stance. To gear for this, wear a shield and a couple tank implants wirh a fortitude stim. Don't drop under 70 percent surge with this setup. Full vindicator set bonus.
I do a lot of ranked PvP so I guess the 2 vindicator pieces are better. Once you explained it I finally understood why people use both set bonus. Cheers