Simple alphabet scrambler --- usage of vectors
Before discussing whether the algorithm is good or not, there are many small things that you could improve to make the code more readable:


Your program won't compile with some compilers if you don't explicitly include &lt;cstdlib&gt; for rand and srand and &lt;ctime&gt; for time.
It is good practice to always fully qualify with std:: the names of the standard library components you use. Therefore, you should explicitly write std::rand, std::srand and std::time.
Don't keep commented out comments. They will hardly help you and will probaby just be in the way. If you need to look again at old pieces of code, revision control software are there for you.
This loop:

std::vector&lt;char&gt; vuser_pass;
for ( decltype(user_password.size()) i = 0; i &lt;= user_password.size(); ++i ) {
    char letter_hold = user_password[i];
    vuser_pass.push_back(letter_hold);
}


If I am not mistaken, you are simply asiigning every element of the string user_password to the vector vuser_password. In this case, please use the std::vector's constructor which takes two iterators:

std::vector&lt;char&gt; vuser_pass(std::begin(user_password), std::end(user_password));


This do what you need and you will avoid having to manually write a for loop. Note that it is also valid for the alpha/valpha conversion.
That said, do you need std::vectors at all? You can as well iterator directly on std::string since it implements the containers methods.
This loop:

for ( decltype(vuser_pass.size()) i = 0; i &lt; vuser_pass.size(); ++i) {
    std::cout &lt;&lt; vuser_pass.at(i);
}


If you use get rid of the vectors and simply use strings, you can directly output the resulting string to std::cout:

std::cout &lt;&lt; user_password;

You don't need to return 0; at the end of main. If nothing is returned, the compiler wil automagically add a return 0; for you at the end. Not wirting it by yourself amy help to document the fact that you won't ever be returning anything else than 0.
You could a range-based for loop to simplify the latest remaining loop:

for (char&amp; c: user_password) {
    c = alpha[rand() % 25];
}

Also, std::rand and std::srand are kind of advised against in modern C++ since there are safer alternatives in the new &lt;random&gt; header. However, the alternative is not that easy to use yet.

std::random_device rd;
std::mt19937 gen(rd());
std::uniform_int_distribution&lt;&gt; dis(0, 25); // will produce ints in [0, 25]


Now, you can create random integers between 0 and 25 by calling dis(gen).
Since alpha is not meant to be modified, it would be better to make it const.




So, let's take all of these modifications into account and write your code again:

#include &lt;algorithm&gt;
#include &lt;iostream&gt;
#include &lt;random&gt;
#include &lt;string&gt;

int main(){

    // random number generator in [0, 25]
    std::random_device rd;
    std::mt19937 gen(rd());
    std::uniform_int_distribution&lt;&gt; dis(0, 25);

    std::cout &lt;&lt; "Welcome, to the scrambler: " &lt;&lt; std::endl;
    std::cout &lt;&lt; "Enter the string (all lowercase letters) you want to be scrambled: " &lt;&lt; std::endl;
    std::string user_password;
    std::cin &gt;&gt; user_password;

    std::cout &lt;&lt; "Select the type of encryption you want: " &lt;&lt; std::endl;
    std::cout &lt;&lt; "1. Standard alphabet scrambler " &lt;&lt; std::endl;
              //Add support for other types...
    int choice = 0;
    std::cin &gt;&gt; choice;

    if (choice == 1) {

        const std::string alpha = "abcdefghijklmnopqrstuvwxyz";

        for (char&amp; c: user_password) {
            c = alpha[dis(gen)];
        }

        std::cout &lt;&lt; user_password;
    }
}


Voilà! Your program has been converted to somewhat clean modern C++. Now, all you have to do is to provide more scrambling algorithms and to provide a mechanism to tell the user that they have chosen an invalid option and that they should chose again.