Is the vault share composable? Can I use it elsewhere in DeFi?

Not yet — and that's on purpose. The share is an ERC-20, but it's currently non-transferable (mint on deposit, burn on withdrawal). That closes a class of attacks tied to moving a fee-bearing share between wallets.

A transferable, composable share (usable as collateral or elsewhere in DeFi) is a future phase — it needs the high-water-mark-on-transfer logic designed and audited first. For now, the share is purely your claim on the vault.