To use Reason's BV512 Vocoder with your own vocals:
1. Record whatever vocals you want to vocode using whatever DAW you have (not Reason because Reason doesn't record audio).
2. In your Reason project, open a new BV512. Hold shift as you create it so it doesn't get automatically routed in a way you don't want it to. Route the outputs of the vocoder to your mixer or wherever you want it to go. Make sure the equalizer/vocoder knob is set to vocoder and the dry/wet knob is all the way wet.
3. Open an NN-XT (again, while holding shift) and initialize patch if it isn't already initialized. Route output 1 from the NN-XT to the modulator input on the vocoder.
4. Open one of the synthesizers (while holding shift) and initialize patch. I like to use Thor but the Subtractor works well also. Don't use the Malstrom as the carrier, at least not at first. Route the output(s) from the synth to the carrier inputs on the vocoder When vocoding it's important to use a synth with an open filter (if not completely open, close to it). Otherwise it won't work nearly as well because there are no high frequencies to modulate.
5. Now load the vocal sample you previously recorded into the NN-XT. Record that sample to a sequencer track. You will not hear the sample at this point because there is no carrier signal yet. Loop the sample and keep it playing continuously.
6. Now if you play the synthesizer, you will hear the sample being vocoded. You're money!
This is the most basic set up for a vocoder. I do a lot of signal processing in between, such as compressing the modulator and equalizing the carrier, among many other things. There is usually quite a bit of tweaking involved until you can get a sound that you like. It helps to put all the devices involved in one combinator to keep things organized.
Hope this helps!