What is the most efficient way to add arbitrary number of elements to an std::vector in C++?

I have a defined class A and an std::vector vec that stores a series of A instances. I attempt to write a function that appends an arbitrary number of A instances to vec.

Should I use vec.reserve() along with vec.push_back() since constantly resizing vector is expensive? Or, should I use vec.insert()?

Currently, I'm not sure if I should design the function to accept a variable number of arguments using variadic functions or a vector of A instances that I'd like to combine and then concatenate with vec?

What is an efficient way to design this function (speed is critical and A is a complex class)?