Vasco

Dont worry, i'll help you out. You can get it by 2 ways, both in chests.

Great Crystal---->Uldobi Phullam Pratii (the area northeast of the second Taurus gate). The path to be open to access the Escutcheon will vary, I would suggest starting from the save crystal just south of Ultima, which would require Taurus Gate II and Capricorn Gate II to be open, as well as either Virgo gate to be open as well. The chest has a 20% spawn rate, and a 50% chance to be the Zodiac Escutcheon, as well as a 50% chance to be aBrave Suit. With a Diamond Armlet equipped, it is a 100% chance to be the Escutcheon. This is a one-time chest for the item, you can only get one or the other. If the chest does not spawn, zone away three screens and return to this area. Please consult the following map to guide you:
Http://www.gamefaqs.com/console/ps2/file/459841/43190

Barheim Passage----> West Annex: Easiest way to do this is to save at Terminus No. 7 Adjunct in a separate save file (so you’re not stuck down here). Before saving, you could buff your characters with Protect, Shell, and especially Float (so you don’t have to worry about traps). From the save point, head east, fleeing all the while. You will run along the thin southern passageway heading east, then head north. The next right you come to leads to a small thin passage heading south (with plenty of traps). At the end of this passage will be a chest. EQUIP YOUR DIAMOND ARMLET. The chest has a 30% spawn rate, and if the chest contains an item, a 6% chance of containing the Zodiac Escutcheon. The overall odds of getting the Escutcheon is 1.8% When you get to this location, there will usually be a chest, because the Escutcheon chest usually sits on top of another chest that spawns 90% of the time, and contains a Knot of Rust or a Sweep. So if you receive an X-Potion or gil, then the Escutcheon chest spawned. If you do not get it, soft reset and try again.

Wow, huge sub, so Hope I Helped