You maybe want to upvote this feature request to build a portable Vivaldi version.
As for the portable use of the standalone version - I use it also for the computers at work so I always carry a thumb drive around with a standalone version. But every time I put a new one on an usb stick I install it on a pc as a standalone in a folder only for this purpose - configure it to my likings and ad µblock origin (as this extension will still work if you copy the whole thing) next I copy it to the stick.
Regarding the "read only" problem your usb stick shows you - is the read only part for the whole usb stick or only for the vivaldi folder on it? If it is only for the vivaldi part this might be the update-notifications that is running. You can disable it in Settings --> Update --> untick the only available option try again do delete