1 Vase 3 Vessels game in C++
A couple of things that could help.

A menu and a switch block will simplify the main function.

A class to represent a vessel with quantity and a state that would be empty, full or partially full, would help clean up the rest of the code.

Make the menu options a '1' or a '2'.  This way you don't have to rely on the case.  

And instead of explaining to the user how to enter the parameters, simply ask them which vessel to pour from and which vessel to pour to, and validate and correct, their answers before calling the functions.

Here's an example of using a class a menu and a switch block.  Some of the functions will need to be fleshed out, but you can easily see how mush easier it is to follow the flow of the program this way:

class Vessel
{
    map&lt;char , string&gt; states =
    { {
        {{'E',"Empty"}},
        {{'P',"Partial"}},
        {{'F',"Full"}}
    } };
    int _quantity = 0;
    int _capacity = 0;
public:
    Vessel( int capacity );
    string GetState();
    int GetCapacity();
    int GetQuantity();
    void SetQuantity( int volume );
    void Pour( Vessel other );
};
void DisplayVessels( map&lt;int , Vessel&gt; vessels )
{
    cout &lt;&lt; "The state of the vessels:\n";
    cout &lt;&lt; setw( 4 ) &lt;&lt; "Size\t" &lt;&lt; setw( 5 ) &lt;&lt; "State\t" &lt;&lt; setw( 4 ) &lt;&lt; "Used\n";
    int counter = 1;
    for ( auto v : vessels )
    {
        cout &lt;&lt; counter++ &lt;&lt; setw( 4 ) &lt;&lt; v.second.GetCapacity() &lt;&lt; '\t' &lt;&lt; setw( 5 ) &lt;&lt; v.second.GetState() &lt;&lt; '\t' &lt;&lt; setw( 4 ) &lt;&lt; v.second.GetQuantity() &lt;&lt; '\n';
    }
}
void Pour( map&lt;int , Vessel&gt;&amp; vessels )
{

}
int main()
{
    map&lt;int , Vessel&gt; vessels =
    { {
        {{24,Vessel( 24 )}},
        {{5,Vessel( 5 )}},
        {{11,Vessel( 11 )}},
        {{13,Vessel( 13 )}}
    } };
    cout &lt;&lt; "Welcome to the Vessel Game\n"
        &lt;&lt; "Three women stole a vase full of balsam(24 oz.)\n"
        &lt;&lt; "During their get away they meet a glass merchant and buy 3 vessels from him.\n"
        &lt;&lt; "Upon reaching a safe place to divvy up their spoils, they discover that the vessels are of odd sizes(5,11, and 13 oz.)\n"
        &lt;&lt; "Your job is to figure out how to divide the balsam into 3 vessels each containing 8 oz.\n\n";
    vessels[24].SetQuantity( 24 );
    char choice;
    bool done = false;
    while ( !done )
    {
        cout &lt;&lt; "1. Pour\n"
            &lt;&lt; "2. Display\n"
            &lt;&lt; "0. Quit\n";
        cin &gt;&gt; choice;
        switch ( choice )
        {
            case '1':
                Pour( vessels );
                break;
            case '2':
                DisplayVessels( vessels );
                break;
            case '0':
                done = true;
                break;
            default:
                cout &lt;&lt; "Wrong choice.\n";
                break;
        }
    }

}