Your usage is wrong. AND-OR lists are not a straight substitute for if statements. They are left-associative and have equal precedence.

This means that your first example is equivalent to

{ [[ "$_config" =~ "vhodddst.conf" ]] || echo "false" ; } && echo "true"

Since the group command returns the status of the echo command, the command returns success and the command to the right of the && gets executed.

In the original example, then, the exit status of the OR list is the exit status of the echo, and, since that is successful, the rest of the AND list is executed.

You should be able to follow the execution of your second example to see why it differs.