You've (properly) set up VISA so that it reads up to a Termination Character, which defaults to \n, <LineFeed>. When you do this, the best way to do VISA Reads is to ask VISA to read 1024 (or some other "too large" number of) bytes. Now, if you want, you can use Bytes Read and process all the bytes all at once. Alternatively, if you are reading a string, you can parse the string however it is appropriate using the String functions.
Something you should always do when starting with a new piece of LabVIEW Hardware is to open MAX and use it to "play" with your hardware so you are sure you know how to do I/O. So for a VISA port, you'd do a Read and see what comes back. A good way to check that you have the Baud rate, parity, termination character, etc.