Also meine Frage ist, wie ich die Lücke wieder geschlossen bekomme, oder welche Alternativen ich habe.

Because vectors keep an array format, erasing on positions other than the vector end also moves all the elements after the segment erased to their new positions, which may not be a method as efficient as erasing in other kinds of sequence containers (deque, list).

This invalidates all iterator and references to elements after position or first!

...

#include <list>

int main()
{
    std::list<int> data;
    for (std::size_t i(0); i < 1000; ++i)
        data.push_back(1);
    std::list<int>::iterator it(data.begin()); // 1. Item
    data.erase(++it); // 2. Item löschen
    data.remove(100); // löscht, wo item == 100
}

usw.