I'm coming back from Europe on SQ and am taking a side jaunt to SGN for a couple of nights.

I'm coming back from Europe on SQ and am taking a side jaunt to SGN for a couple of nights.

Am I able to leave my big luggage at the airport in Changi for a few days in a locker whilst in SGN? I'm only going to take a small overnight bag with me.

Anyone have any experience doing this in SIN?

I just don't want to have to lug my bag around more than I need to.

Check out the Changi Airport website. The link details available luggage storage services and costs here:

http://www.changiairport.com/en/airport-experience/attractions-and-services/baggage-storage.html

Honestly I wouldn't bother with the effort..... You will catch a taxi anyway as they are so cheap so don't take the MRT... Plus if you have a late flight make sure you ask for a late checkout, SG is geared for late check ins so getting a late check out is easy. 

By SGN I meant Ho Chi Minh City in Vietnam.

Im wondering if theres any issue with leaving the country with stuff in the locker 

If you are truly concerned with the issue of leaving stuff in the locker and leaving the country temporarily, I would visit the link I supplied above and whip off a quick Q&A email to them.

However, since Singapore is a relatively small country and a regional major business location, I'd imagine that this scenario is fairly commonplace. Just make sure that whatever you're storing is legal & inoffensive, so that no questionable circumstances arise.