One can write a string vector to stderr using 2 and -2. The difference is that -2 appends a newline after the string, whereas 2 does not. Note: this expression has a return value of 2 or -2, respectively, which will print as well (to stdout) if not suppressed.
q)2 "String vector here\n" String vector here 2 q)-2 "String vector here" / equivalent String vector here -2
This is a special case of send to handle.