The information I know, coming from Hikvision is that the problem with the missing e-mail snapshots is related to a shortage of RAM in the NVR. That issue should be fixed in the latest firmwares only for the motion triggered events, while for the VCA triggered events wouldn't be possible.
However, I'm little bit sceptical about fully resolving that issue. It sounds me like the pre-recording setting in the DVRs/NVRs where you can set up to 30 Seconds pre-recording time in case of event for each channel. But in reality you get no more than 0 to 2 seconds pre-recorded time, because the hardware buffer holding that information is getting filled almost immediately.
And if if you ask Hikvision: why I set 30 seconds , but I get only 1 second, their answer will be: lower your main stream resolution to CIF, 128 kbps, and you'll get 30 seconds for all the channels.