According to CNET's Scott Stein, the last demo during the HTC Vive demonstration was:

Finally, I ended up in an all-too-familiar familiar antiseptic white room. A robotic voice addressed me, and yes, I was in Aperture Science, being dragged through a robot repair program educational trainer that, as you'd expect, doesn't go as smoothly as expected. It looked incredible: the writing and pacing were perfect. It felt like a Disney ride. I couldn't stop grinning. Portal 3, perhaps? Sounds like it.

Engadget's Nicole Lee also had a more detailed report about the Aperture Science demo, which she describes here. EDIT: I removed the pasted paragraph since some people might find it a spoiler, in-case the demo is released online and they'd rather see it first. Still, you can read the full thing at that link.

There are similar reports from others, including this person who thought it was Portal 3:

The guy from the video clearly jumps the gun, however, as it was just a demo teaser and it in no way represents a full-featured new game from Valve. That said, it'll be interesting to see if anything is announced at Valve's conference tomorrow - which focuses on "Vulken" (aka glNext) and could go anywhere from there. We already expect to see Source 2 footage running on glNext tomorrow, maybe this Aperture demo also serves as a tech demonstration for the new engine?

Personally I doubt we'll get any new games from Valve at GDC. The Aperture teaser was likely just there to show off what virtual reality might look like when it's being used by a familiar game, and Half-Life 3 is likely still in development hell and Gabe lies awake every night trying to figure out a way to break it to the fans without sparking a government overthrow. Still, there's always that dim hope... right?

UPDATE: PCGamer now has confirmation from Doug Lombardi that they did not announce Portal 3 through that teaser and it was purely a tech demo.

I still really want to see this demo in action. I hear it's hilarious.