you might need to cast i to a string (maybe the compiler will do it automatically for you but I don't think so) first, and a vector of strings is similar to a 2D char array so you need to specify which string from which vector you want to add to, but then ya, the code you have should work. It will append the char from the string in vector A to the end of the string, although there is an insert function to place the new char somewhere other than the end if you wish.
vectorB[3].pushback(vectorA[8][7];
add the eigth char in the ninth string from vectorA to the end of the fourth string in vectorB.
