How can I get a field from the last element of a vector in C++?

The immediate answer to your question as to fetching access to the last element in a vector can be accomplished using the back() member. Such as:

int var = vec.back().c;

Note: If there is a possibility your vector is empty, such a call to back() causes undefined behavior. In such cases you can check your vector’s empty-state prior to using back() by using the empty() member:

if (!vec.empty())
   var = vec.back().c;

Likely one of these two methods will be applicable for your needs.