Those are not the Vortex PBT keycaps. They had a similar font, but the escape key was labelled "Esc". It looks very similar otherwise, and the Vortex' were available in that color combination.
One way to tell is the color of the white. IMSTO "white" keycaps are a very distinctive gray. Vortex keycaps are much more of a bright white. From the picture, these look too bright to be IMSTO's.
- Ron | samwisekoi