Vivosmart 5 mileage is way off

Are you walking with or without GPS?

Having location enabled has nothing to do with distance walked.

The Vivosmart 5 uses Connected GPS technology for activity tracking, information on Connected GPS can be found in - Using the Connected GPS Feature With a Garmin Watch.

If you are tracking the activity using GPS than the distance data is generated via GPS tracking of your route. 

If it is not using GPS data than it would revert to the accelerometer counting steps and Steps*Stride length=Distance

Which method are you using? what are you seeing that tells you the distance is wrong?

I've tried both. Changing the number of steps per mile does nothing to change the results that I'm getting. It invariably doubles my mileage no matter what I enter for stride length. Lately, I've been using the activity tracker. That's more accurate, but it still adds some mileage. 

Can you provide dates and activities where the distance was wrong, with what the distance showed on the watch versus what it should have been?

May I have your permission to review your Connect data?