To find out how many vanilla cupcakes were sold, we first need to determine how many mocha cupcakes were sold.
Given that 77% of the 200 cupcakes sold were mocha, we calculate the number of mocha cupcakes as follows:
\[ \text{Mocha cupcakes} = 77% \text{ of } 200 = \frac{77}{100} \times 200 = 154 \]
Now, to find the number of vanilla cupcakes sold, we subtract the number of mocha cupcakes from the total number of cupcakes sold:
\[ \text{Vanilla cupcakes} = \text{Total cupcakes} - \text{Mocha cupcakes} = 200 - 154 = 46 \]
Thus, the number of vanilla cupcakes sold that day is:
\[ \boxed{46} \]