It's not the [metro] that is shaky in your patch, it's the
[impulse~]! Attached variation proves this.

Pd's messages come in two kinds: regular, GUI-generated messages and
so called "clock-derived" messages. The time objects in Pd like metro,
delay, pipe etc. all generate clock-derived messages. Their timing is
not affected by dsp-blocks at all.

However, and now comes the important trick: DSP objects need to do
some extra work to make use of the accurate timing of clock-delayed
messages, when converting these messages to audio signals. So far, the
only object that does this is vline~ AFAIK. You may want to have a
look at its source to check out what needs to be done to convert from
control messages to signals with accurate timing.