Originally Posted by Goldorak

Yes, on-line and, if I remember well, you receive a QR code that you scan at an automated gate with your passport


So, you're fine.
My personal experience a few months ago : I was in Sao Paulo and was going to Singapore via Paris (short stop at home) with 3 days between leaving Brazil and arriving in SG. Brazil is also on the SG list of countries affected by yellow fever (even if there's none in SP), so I got the vaccine 1 month before going to Brazil just for the purpose of the trip to SG. On the on-line SG arrival declaration, I declared that I visited Brazil. When I scanned the QR code at the gate, a red light beeped and I had to see an officier and show him my vaccine certificate.
The vaccine is for life (one shot and that's it), so this is also something you may consider in the future.