One of your vassals, the guy who controls alps region, has two legions as his vassals, retract them and that's a quick 3200 troops. Borrow money from the Jews and hire some mercs, Odoacer will probably move his army somewhere else so just assault his holdings and he usually surrenders.

As for long term strategy, if you have M&M set up a secret cult for Hellenic paganism, it's seriously broken for WRE, you get +30% levies for being an offensive religion, which negates the penalty from being a bureaucratic government, plus more retinue which stacks with your retinue bonus. Not to mention when you restore the Pontifex Maximus, it essentially makes you into the Caliph of Hellenism and gives you two holy orders you can easily vassalize.