how to use mysql temporal cache tables
If I understand your question correctly, your examples are just storing the vcount of visitors (visitors_stored) from today.

This means 


You'll want a script to reset the visitors_today each day at midnight.
You'll want a trigger on visitors_stored to update the visitors_today table.


Why would you do it? If you regularly wanted to know how many visitors were from today, you would do a 

SELECT vcount FROM visitors_today


instead of 

SELECT v_count FROM visitors_stored WHERE vcount_date=NOW(); -- Probably wouldn't use NOW, but an actual static date!


It depends on how your server is tuned, but generally if you have an index on the vcount_date column in visitors_stored, it will suffice. If your visitors_stored table was overly large (>100GB say), the cache table might be of use. 

If you start to see performance issues with computed columns, a cache column or table might be the way to go.