Hashicorp providing official binaries for arm64 since 0.7.1 release; I managed to build latest version from Gentoo tree (1.10.3) on my aarch64 vm without additional patches. Vault have one dependency that missing ~arm64 (dev-go/gox) which is building fine on aarch64 too.
Sanity check failed: > app-admin/vault-1.10.3 > bdepend ~arm64 stable profile default/linux/arm64/17.0 (8 total) > dev-go/gox > bdepend ~arm64 dev profile default/linux/arm64/17.0/hardened/selinux (1 total) > dev-go/gox
Sanity check failed: > app-admin/vault-1.10.4 > bdepend ~arm64 stable profile default/linux/arm64/17.0 (8 total) > dev-go/gox > bdepend ~arm64 dev profile default/linux/arm64/17.0/hardened/selinux (1 total) > dev-go/gox
arm64 done all arches done