test.zsh

#!/usr/bin/env zsh

# Emulate Vim executable that simply returns its arguments.
> /tmp/vim.rb <<EOF
#!/usr/bin/env ruby
p ARGV
EOF
chmod 700 /tmp/vim.rb

VISUAL="/tmp/vim.rb -c 'first arg'"

bad() {
  ${=VISUAL} -c 'second arg' -- $1
}

good() {
  eval "${VISUAL[@]} -c 'second arg' -- $1"
}

bad file
good file

$ /tmp/test.sh
["-c", "'first", "arg'", "-c", "second arg", "--", "file"]
["-c", "first arg", "-c", "second arg", "--", "file"]