If the two vectors have the same size, in a while loop, test that each successive pair of elements is equal.

bool is_equal(vector<int> a1, vector<int> a2)
{  
   if (a1.size() != a2.size()) return false;
   int i;
   for (i = 0; i < a1.size(); i++)
      if (a1[i] != a2[i]) return false;
   return true;
}

Use the assignment statement.

   vector<int> a(2);
   vector<int> b(4);
   . . .
   a = b;

Assign 0 to each element in the array.

  for (int i = 0; i < a.size(); i++)
     a[i] = 0;

Assign an empty vector of the same type to an existing vector.

  
  vector<int> a(2);
  . . .
  a = vector<int>(); // empty a