Just a proposal: to ease the script writing it could accept input from stdin.
Your example could be more readable written in bash like this (uses bash heredoc syntax):

SESSION=$(vycli setupSession)
vycli --session=$SESSION --read-stdin << EndOfFile
configure
set "system host-name vyos"
delete "system name-server 192.0.2.1"
commit
exists "service dhcp-server"
commit returnValue "system host-name"
EndOfFile

vycli --session=$SESSION --format=json show "interfaces ethernet"

Also, you could save a file with the commands and do this:

vycli --session=$SESSION --read-stdin < myfile
vycli --session=$SESSION --format=json show "interfaces ethernet"