If your question is "Can I reanimate old smartphone to use is again?" The answer is not easy.

Ubuntu Touch and Lineage OS support only modern devices. There's postmarketOS, but many features don't work on this OS. You may search for custom ROM for your device, but this is not easy. You don't say what device you use.

You also probably want to ask "Will Vivaldi team support this old devices?" It's hard to answer this question, because Vivaldi is heavy dependent on Chromium project. You need Android 8.0 to install recent versionof Vivald. Firefox require Android 5.0. The best option for you it to buy cheap used phone with Android 8.0.

What to do with old phones? Vivaldi team can use their coding skills and create software for old smartphones:

NAS, FTP, backup server Opera mini compression server fire detection water detection smoke detection thief detection smart lights turn on/of (movement, schedule or daylight activated) peephole (entryphone) walkie talkie (skype) on short distance internet or fm radio controlled by bluetooth fancy clock or weather station smart electrical circut controller smart radiator controller personal VPN cache proxy server cache DNS server TOR server dashcam mp4 video recompresion on real time wifi 2 bluethooth extender printer server buildin temp monitor (water freezing detector or fire detector) http serve (ddns) e-mail server (ddns) e-book audio synthesizer photo NAS like dropbox flashlight activated by movement flashlight activated 10pm - 6am

Screen time tracking for ios and android