I'm a bit surprised by the scope of the quotation marks.
Indeed, it looks like
set set_default_mouse1 bind MOUSE1 "+attack; +vstr low_sensi high_sensi"
has the same behavior of
set set_default_mouse1 "bind MOUSE1 +attack; +vstr low_sensi high_sensi"
The error I got on the console says that "+vstr" is called outside of a key-press/release, which means it is called with as a second instruction of the "set".
Do you see any way to enforce the expected behavior?