Videos are photos with two extra problems: they are large, and they must play without being decrypted to a file first.
The Hidden album
Works for videos exactly as for photos. Same limits: not encrypted, still in iCloud Photos, open to anyone with your passcode.
A vault app, done right
Look for two things. First, the video should play straight from the encrypted file. If the app writes a decrypted copy to play it, that copy can be left behind. Vaultpod decrypts one chunk at a time as the player asks for it; nothing unencrypted touches the disk. Second, check what backup does with videos. iCloud’s free 5 GB fills fast with video, so the app should say plainly when the backup is paused for lack of space rather than failing silently.
Free limits
Most vault apps cap free videos lower than photos because of size. Vaultpod’s free plan holds 15 videos; viewing and exporting are never limited, only adding more.
Delete the originals
After import, delete the originals from Photos and then from Photos’ own Recently Deleted album, or the video is still on the phone twice.