Using my tutorial will give you the solution you are looking for; however, I'm going to list another for your solution here. There may be another way to do it, but this is the easiest method that comes to mind. I know you don't want another number, but read this through before ruling it out because you will end up with a Google voice number but it wont be used...
Using a computer: Go to
www.google.com/voice an sign up for a phone number. Choose one or get a random one. The number doesn't matter unless you plan to use it.
The rest of this is done from your phone: Once you are signed up for the number, follow my tutorial EXCEPT FOR STEP 1. DO NOT TAP ON "SKIP" FOR THE FIRST STEP. Then go through all the prompts as listed. There will be a step where it will ask which number you would like to use as your number. Choose your sprint number. Now your Google voice number is dropped and you have Google voice as your VVM service!