It's clear in the system.
Ticket 1 PNRABC1 GLA-LHR-GLA
Ticket 2 PNRXYZ2 LHR-VIE-LHR
Each is a different booking with a different PNR. They won't tag through the bags sadly.
They will only tag Ticket 3 PNRKLM3 GLA-LHR-VIE-LHR-GLA as that is a protected through ticketed connection.