'Transiting' at HKG - quick question...

This may be a stupid/easy question, but not having done this before, I thought I'd seek the advice of someone who knows HKG.

I'll be transiting through HKG in January, from VS inbound, to an SQ flight to SIN. Seperate bookings and PNRs, and about 4 hours between scheduled times.

On arrival, do I need to go through immigration (marking my card as a stay of less than 1 day) collect my bags, and walk back around to SQ check-in, and proceed as normal from there? Or is there an airside way of transiting between different carriers?

You will have to get your bags and go through immigration and then back in again. The VS staff can't easily check your luggage through and won't be able to issue you a SQ boarding pass.
If you can get them to check your luggage through then you would only need to go to the transit desk at HKG and avoid going landside.