What rate of frame insertion is acceptable?
I have a VHS-C I am running from Sharp SVHS (VC-S101) to an ES15, to Hauppage 850, and for 30+ plus minutes of playtime I often get as many as high teens to low twenties for frames inserted, no dropped frames, though. Is this an issue that can be ignored?

It means that Virtualdub inserts extra frames to compensate for a long delay between frames arriving from the capture device or similar. You can tell virtualdub to not do this, but if it happens it usually indicates an issue. The ES15 should deliver a stable frame rate out, so maybe some other process is using up system resources. Make sure there isn't some antivirus, search indexing, updates etc running in the background when capturing. Setting the process priority of virtualdub to high or realtime (the exact method depends on what windows version you are using) may also help.

First gen i7 lappy, this is my second tier setup. Antivirus is off, preview off, probably better to add a second internal drive, I’m capturing to C desktop. I tried an external drive but it was inserting frames at a craaazy level. I’ll look at the other things you mentioned.

Same as the drop rate: none, or close to it.

I monitor captures. If I see 1 dropped frames between check-ins, I'm fine. If more (5-10), I'm not, check-in more. If I see more than 10, I stop the capture and review it. In most cases, HDD fragmentation is the issue.

Yes!

Yikes! That's a major no-no.

That's almost always another no-no.

I moved the capture location to C drive CAPTURE, shut off the preview, shut some things like internet and anti-virus off, increased the video buffer in VDub, and there was improvement, but it's a lousy tape. Without a second ES-15 or a second TBC-1000 (thank you LS), most of my captures will need to be one at a time. I'd love to get another ES-15 but if I plug one more device into my electrical outlets my neighbors will have a brown-out.

My current theory is that "frames inserted" is caused by the VCR not pulling the tape across the heads at a constant rate, perhaps the tape is warm and prone to stretching. After I complete my capture, I fast-forward the tape to the very end, rewind back to the very beginning, then take the tape out and turn the VCR off and let both cool down for a few hours before capturing again using the same settings. Then the second time it often captures with 0 frames inserted.