VSRE stands for "very short reply expected".
It's a way for you to indicate to the recipients of your email that they have to reply with a short (usually one to five words) reply.
Typical replies to VSRE are "Yes", "No", or "Tuesday", or “Pls send more info".
It is not a standard, just a human convention, like FYI, or RSVP.
We all get a ton of email. In most cases replying politely can be demanding.
But it shouldn't be.
When someone sends you an email with "VSRE" in the subject or the body, he/she is expecting a very short answer.
"Expected" or just "Welcome"?