From https://grapheneos.org/usage#web-browsing,
Vanadium will be following the school of thought where hiding the IP address through Tor or a trusted VPN shared between many users is the essential baseline, with the browser partitioning state based on site and mitigating fingerprinting to avoid that being trivially bypassed. The Tor Browser's approach is the only one with any real potential, however flawed the current implementation may be.
It seems like spoofing is not very effective against fingerprinting techniques given that the devs don't consider it as an approach with real potential. Instead at https://grapheneos.org/features#vanadium they say
Anti-fingerprinting depends on having a large userbase with the same browser, extensions, content filters and other web-facing configuration.
Also display info is needed to display web pages properly; if this is spoofed pages would display strangely. The closest to this is Tor's letterboxing, though it would be unreasonable on small phone screens. Vanadium users have very similar display and hardware info anyway since it's only available on pixels, so spoofing may not be necessary. See the GrapheneOS features and FAQ pages for Vanadium for more info on anti fingerprinting.
Spoofing OS would probably not work given that Vanadium is only on Graphene OS, so if a site can identify that the user is on Vanadium, they can tell they are using a pixel with Graphene anyway.
IIRC Brave uses spoofing in their anti fingerprinting measures, and it only fools fairly basic scripts with the real fingerprinting threats still being able to work without issue.