You could use Python for exporting the exact format you asked for.
(what are a, s, and t though ?)
(I guess a is for alpha, but I don't think VertexColor data has an alpha channel. For s and t, I dont't know. Your question is not clear on these points).
If you accept other similar format, then you can export as PLY :
Don't forget to activate "Vertex Color" :
You get such a file :
The header explains the structure:
The orange line tells you you'll first have 1966 lines desribing the vertices (what you want) with columns described in yellow.
The pink tells you that you will have 500 faces with the property in blue. You didn't ask for it though. But it seems you cannot bypass them at export time.