I'm pretty sure SQL Server and MySQL use locking instead of MultiVersion Concurrency Control so they don't keep more copies of data around. No vacuum needed but there's a possibility of things blocking.

But I might be out of date.

Yes, MySQL has read locks.

But I don't believe SQL Server does.

InnoDB (MySQL's default storage engine) implements MVCC using undo logging and background purge threads. It scales to highly concurrent OLTP workloads quite well. It doesn't work well with OLAP workloads / long-running transactions though. The oldest active transaction will block purging of anything newer than that transaction's snapshot.

SQL Server has had MVCC since 2005.

Yes, but doesn't it require opt-in to enable snapshopt isolation? Most T-SQL devs will probably default to locking (TABLLOCK, etc) becuase that's what the bulk of google search results for "how do I fix my broken query?" tell people to do: it's only very, very rarely do I see a stackoverflow or dba.se answer that mentions MVCC-related topics.

That's because they don't store non-current versions of rows in the table itself, so why would they need a vacuum? MySQL does need to vacuum indexes, however.