Does the vivoactive have a ecg function? And apple watch comparison?

No.

Do you know why it doesn't have it, or if it will coming in the future? 

I don't know either of those sorry, but Garmin's target market is fitness so perhaps not such a priority?

No, because Apple which is a Smart Watch has far better wrist HR sensor than Garmin which is a Sports Watch brand ! In order to have a FDA approved ECG function, you need to have an accurate HR sensor first !

I don't really care if the ECG is FDA approved.  The Vivoactive 4 is reported to be pretty accurate, so I could send an exercise ECG to my doctor.

I don't think it will be possible without FDA approval and Garmin won't add this feature to it's existing devices, may be to Fenix 6 line only. Yes, HR sensor is accurate most of the times but still behind Apple. There are many criticizing videos from Fenix 6 owners on youtube regarding HR accuracy.

My wife has an apple watch and I have a vivoctive 4S.  We walk / run about a mile / day and they are both always within 2-3 bpm of each other and the gps is within 0.1mi of each other.

Do you mean when one of you wears both devices, or each wears their own? If the latter - you mean you and your wife have the same heart rate!?!?

HR sensor is genereally accurate but sometimes it becomes full of crap. Cardio mode immediately increases HR even though i stand still. Running and strength mode is good most of the time. But Apple is still more reliable.