On Eureka glowy armor, as far as I remember, all you need to do is progress your weapon to the Pyros level(so you need to do Anemos and Pagos and the introduction quest in Pyros). This unlocks the vendor that sells the armor, need some Pyros materials to buy it though so you have to grind that part too.
Then when you get to Hydatos(which doesn't require the weapon, just max lvl and doing the quests), the upgrade vendor is available even without the weapon quest progressed that far, so all you need at that point is just Hydatos mats to upgrade it.
You will also need to complete the Logograms book thing to unlock said vendors(basically same as Bozja Lost Actions, you buy stuff from the AH, open them, combine them to make special actions useable in Pyros and Hydatos and you get all of them to unlock armor vendors, check a guide).
It is quite a process to just get the armor, and if you're there you might as well consider getting a weapon while you're at it, as long as you like how it looks. Eureka weapons were kinda meh imo, but a few stand out as pretty good(DRK and RDM iirc).
