visitor_id not available in datafeed, told to use 'visid_high', 'visid_low' 99.85% of rows have visid_high = 0, visid_low = 0

Did you check the post_visid_high and the post_visid_low?

Are you using data ingestion? Or do you have processing happening where the visitor ids are being set in that stage?

What about mcvisid?

​@donusarmy maybe mcvisd column could help
 

Use concat(post_visid_high, post_visid_low) if you want visitor id. Legacy visid_high, visid_low columns might not have data.