But market cap is price multiplied by total outsanding shares ( public + privately held shares ).

If you and I IPO a company. We each keep 25% of the shares and float the remaining 50%. Using your logic, our insider shares representing 50% of the company has no value.

Also, if the company were to pay dividends, are our shares not applicable?

You can't calculate the value of a company on just the float, unless the float represents all or most of the shares.

Of course having such a small float is ridiculous as well. Seems like the vinfast stock is exists solely for financial shenanigans behind the scenes.