To use an overdot as a legend string with TeX interpreter, use a unicode diacritic character (see resources below).
For example, I copied the unicode characters V̇ and Ṫ from the links below and attained its numeric representation in Matlab using,
double('V̇') % Copy-pasted character
double('Ṫ') % Copy-pasted character
Rather than storing those characters in your code, just use the numeric equivilants.
Convert back to char and assign the value to the DisplayName properties of the graphics objects that should appear in the legend. The 2nd line shows how to combine the overdot character with other characters.
hold on
h(1) = plot(0:.1:1, rand(1,11), 'DisplayName', char([86 775]));
h(2) = plot(0:.1:1, rand(1,11), 'DisplayName', ['(' char(7786) 'G)/X']);
lh = legend(h, 'FontSize', 14);
lh.Interpreter
Resources: Overdot characters
