You specify the type of the allocator as a template argument and, if you don't want a default-constructed one, a value as a constructor argument:
std::vector<int,MyAlloc<int>> v((MyAlloc<int>(a)));
Note that I added an extra pair of parentheses to avoid the "most vexing parse". In this case, we can't avoid that using brace-initialisation, since that will try to use the initialiser list to populate the vector.