The string specified in a call to v_stcpy() must be large enough to hold the character contents of the portion of the window read out plus the null terminator. For v_vstcpy(), the string must be large enough to hold the characters and their associated attributes. To do this, you must (1) determine the number of bytes necessary for the string and (2) then allocate the required memory from the heap using mem_get().
mem_get() is the standard Vermont Views memory allocation function. It takes as an argument the number of bytes to allocate.