Not trying to get paid twice, but which will have precedence if someone accidentally upgrades an old link with the ?ref= part still in it?
I think both(vanty and ref) link will work if someone use both then only one will be counted because when any click will done then will check buyer and referall account and only for the first action (purchase) will be counted for commission.
Once the legacy links are deprecated which will be at the end of September, the legacy links will no longer work at all (even on linked accounts). We will be sending out comms about this later this month to allow at least 60 days to update all legacy links.
I would advise you change all legacy links to Impact to avoid the risk of missing referral payouts.
Happy to provide assistance as always,