If you’re not using https, then anyone who can see the traffic in your VPN can see everything you do.
Whether that’s a problem depends. In principle, as long as access to your VPN is restricted to people you trust, then http inside it is fine. In practice, if your security isn’t perfect, and someone gets access to your VPN, then https provides a meaningful second line of defense.
My recommendation would be to never use http for anything where a breach would be really painful. It’s good that vaultwarden is on https. Beyond that, https is absolutely the best practice, but if someone getting access to your service is only a mild inconvenience, then leaving it on http is fine as long as it’s only accessible inside your VPN - the risk is relatively low, and the consequences aren’t bad.






Tailscale is a sort of VPN. So, yeah, treat it the same.
I’m mostly responding to add a little context to the security concerns. Probably the biggest inside-the-VPN concern is that someone on your VPN gets some malware. So, if this is your family network, and say, your kid’s phone or laptop gets some malware, now any non-encrypted traffic is potentially compromised. Basically, the VPN is only as secure as the weakest link. That’s the thing you’re protecting against with https for individual services.
If I were hosting vaultwarden, and had my bank login in there, I absolutely wouldn’t consider a VPN + http sufficient protection. With LLMs, if we don’t have malware that knows how to hunt around for valuable credentials in the wild already, it’ll be here soon. I personally run my Jellyfin server over https because I’ve gone to the trouble to set up a good system to use letsencrypt with nginx as a proxy for any service. But I wouldn’t be too worried about running Jellyfin over http.