See https://docs.google.com/document/d/1Bz5-UB7g2uPBdOx-rw5t9MxJwkfpx90cqG9AFL0JAYo/edit for details

The new semantics changes the meaning of (breaks) source trees containing directories already named “vendor”. Of the over 60,000 listed on godoc.org, there are fewer than 50 such examples. Putting the new semantics behind the environment variable avoids breaking those trees for now.

If they did the analysis right, we would be one of those 50. I can do this soon, it will be pretty easy. We can't use the new go vendoring experiment yet and should stick with the party tool as using the new vendoring experiment would break compatibility with 1.3 and 1.4, but I would propose external instead of vendor, thoughts?

This is minorly urgent as 1.5 is due in about two weeks.