It depends completely on who at what position on the singing order on Sat night. If they are at the end and we're talking about Lorna/Luke/Shelley/Sam C/Rough Copy/Kingsland/Miss Dynamix and they do a decent vocal/amazing vocal then they'll get through to next week. But if any of that lot are at the beginning end of the show then they're screwed no matter the vocal I guess, especially Lorna/Luke who haven't had that much airtime.
I think the only acts that are completely safe this week will be the girls, Nicholas and Sam B.
If I had to predict right now, I'd say for the moment it'll be Shelley and Luke because I think Lorna will deliver a good enough vocal - but we'll have to see.