Approach to take regarding a field that is frequently re-computed
I would add another table for versioning of data. It would have composite key of data and timestamp or version number and in table I would hold actual data.
When new data arrives in table data is updated and in versioning table is inserted.

Regarding vacuuming you set auto vacuum for that table.
If table gets bloated despite auto vacuum setting you can run vacuum full,
but it would fully lock your database.
If you are running on linux you can use pg_repack extension.