I've wondered about this for years and now am at a point where I need to turn a VST effect on at a certain point in the song and then have it remember that so when I rewind it's off and then comes on at the point where it's needed...I've automated volume and pan a bunch in the past, but never could figure out why that little blue on/off button on all your fx in the fx bins doesn't react to write automation? Something so simple...

At least, this was the situation some years ago. I don't think it has changed:
On/off automatibility is VST dependent. AFAIK most VSTs don't support it. I don't know why.
The workaround is automating the actual VST settings between neutral/desired settings or using a send and automating that.
 

Kalle Rantaaho
At least, this was the situation some years ago. I don't think it has changed:
On/off automatibility is VST dependent. AFAIK most VSTs don't support it. I don't know why.
The workaround is automating the actual VST settings between neutral/desired settings or using a send and automating that.

Incorrect. It depends on what DAW you are using.  In Studio One every VST either native or third party has bypass fully automatable. Had it for years. For some reason Sonar cannot do it. To be honest I rarely use it though but I am sure in some situations it could be handy.

You could also, put the effect on a send and automate the send.

dwardzala
You could also, put the effect on a send and automate the send.

 
I tried that. The effect is not as pronounced as it is as having it right on the original track. I'm stunned something this simple cannot be automated ON/OFF on the track!

You could also duplicate the track.  Insert the VST on one, and then delete the unwanted sections of both tracks.

Or use the clip FX bins or if the plug-in has a VST3 format version use it or find a VST3 replacement. VST3 plug-ins have a host bypass function.

This may not be the same for what you need. But close to it, what I do is 'split' the clip at the point you need the effect. Then, 'insert' the VST "only" on the split clip.

Sorry, Duplicate

You can do this with some of the FX.... I recall playing around with automating the FX..... it involves using an envelope to control the on/off and doing it with midi. I had to set up a midi track, insert an envelope and link it to the FX in the audio track.  Seemed like a lot of work to turn something on and off.
 
The easier way is to simply clone the track.... place the FX in it at the level you want, and use volume envelopes to switch between the 2 tracks. Zoom in to set the envelope on in one track and off in the other at the same exact time.  If the track is... for example a guitar or a vocal, it's really easy to get those edit points spot on.  Make the envelopes switch in the pause between the phrases.
 
I find this method to be so much easier and quicker to implement than the time it takes to set up the midi control envelope and link it properly to the FX in question.