Are elements of a 'const std::vector<:string>' immutable as well? (in C++11)

This will compile:

std::vector<std::string> test{"hello"}
std::cout << tt.[0] << '\n';
tt[0].append(" world");
std::cout << tt.[0] << '\n';

Output:

hello
hello world

This will not:

const std::vector<std::string> test{"hello"}
std::cout << tt.[0] << '\n';
tt[0].append(" world");
std::cout << tt.[0] << '\n';

More importantly, as expected, this will not compile:

std::vector<const std::string> test{"hello"}
std::cout << tt.[0] << '\n';
tt[0].append(" world");
std::cout << tt.[0] << '\n';

but sadly, neither will this:

std::vector<std::string> test{"hello"}
std::cout << tt.[0] << '\n';

There are many differing answers to questions about immutability of std::vector and details on cppreference.com (Assignable, CopyAssignable...). But getting down to what I'd like to be able to do (so the question is: "how do I do this..."? --

1. define a std::vectorstd::string that will allow me to ensure the elements, once added, cannot be changed -- but the vector can be changed, including adding new and deleting existing elements and changing their order (std::vector failed though seemed a reasonable approach without understanding the details of std::vector)
2. define a std::vectorstd::string that will not allow me to change the vector or its elements (const std::vectorstd::string seems to work)
3. define a std::vectorstd::string that allows the existing elements to be modified but the vector itself will always have the same number and order of elements (this is not particularly interesting to me)
4. std::vectorstd::string is fully modifiable.