I'd live without the performance data personally.
You could try a restore, remove the new VM's from the inventory, do a VC DB restore, then re-register the VMs you removed.
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The only way I know of would be to restore the SQL database. You can only export the performance data through the VI client not import it. This is probably going to be more work then it's worth. The data you want to restore is in the VPX_HIST_STAT table.
Here's some documentation I did on the database...
SQL restore links...
Thanks guys. I was wondering about restoring the old backup to a different db, export the tables and them importing them into the current db. I think I'll just live with it as a reminder to check those maintenance plans more carefully.