You have a couple choices there.
One would be to redirect stderr of the command, and wrap your close in
a catch:
set vmstat [open "|vmstat -n 60 2> /dev/null"]
...
catch {close $vmstat}
However, the redirect above is Unix specific (might work on MacOS,
likely won't work properly on windows).
Your other option is to (assuming you have Tcl 8.6) use the chan
command to create a pipe, then use exec and output redirection to send
vmstat's output into the writer side of the pipe and read the reader
side in Tcl. In this case exec will return the process id of the
vmstat. You'd save that somewhere and use it to kill the process later
(which would mean either installin Tclx, or exec'ing a "kill" (which
would be Unix specific again).
Presuming you are ok with Unix specifics (all your items seem to be for
managing Unix/Linux systems so far) then the redirect in the open plus
the "catch" method is by far simpler.