The vulnerability spell is an option for magic over using vulnerability bombs, It is currently a pain to use at the best of times. Often they just don't work and disable you from doing any damage or using any abilities until a normal spell is cast while using spell book swap.

In the video attached, I spell book swap no the normal spell book and vuln during a phase transition - nothing wrong there. I am then unable to cast any normal spell after multiple attempts - I repeat the spell cast and target selection many times.

Vulnerability should be cast on next global cooldown and cleanly cast, this is not acceptable for a spell released soon to be 20 years ago.