Is eighth the highest level of vestige or is it nine like everything else? If all you need is another four levels of advancement legacy champion can do it while technically only losing one level of spellcasting by going to the fifth level of the class. I say one because if you're willing to be properly cheesy anima mage actually doesn't specifically call out an advancement of existing binder features the way most theurgy classes do. You can get the entire benefit of it entering with feats.
Eighth is, it's one of the weird things about Binding which make vestige progression wonky and difficult to balance. So yeah Legacy Champ ought to work*.
*I've only read like 15% of Weapons of Legacy before deciding it wasn't worth the time required to read it and thus haven't read Legacy Champ in its entirety.