No way to make it true stereo. You can fake it though. If you put a very quick delay on your voice track. (like 50 MSec or less) and only one repeat, then 'Pan' that signal to the opposite side. (If your mic is on the left channel, pan the delay to the right channel only) This will fatten your voice a little and not make it sound so flat. It has to be a short delay or it will sound like an effect. It won't sound exactly like stereo, but it won't sound like mono either.
If you look at the audio effect called "Simple Delay", there's a setting in there that's called "Psuedo Stereo". I've never used this and can't describe how to set it up, but if you note that the delay time is set to 0.010 (10 MSec.) If possible, I'd set it up so that the original voice is panned to one side and the simple delay is panned to the opposite and make the levels sound as if it's fairly centered.

As mentioned a VO is a mono signal. When you record, you should be selecting one channel/mono recording on your sound card. Recording in Stereo will give you the current results you're seeing where there is nothing on the other channel. The channel switch like mentioned should overcome this. You actually want a dry/bland sound when recording a voice over. Using the simple delay probably isn't your best choice as an effect. You can actually run into phase problems, which can make the voice sound thinner when using this effect. The better approach would be to add a small room reverb to give it some life and in addition to that add a compressor and some EQ if it's needed. It's probably best to create an FX's bus with the reverb on it and put the EQ and compressor in the channel insert of the VO track. The reverb FX is actually a Stereo FX. When you send a mono signal to it like your VO, the reflections will be different in the output of the reverb, creating a stereo sound. If you set up an FX's bus, you will send the dry VO to the reverb by using the FX send, which will be located under drop down selection of the Pan control on the track fader controls. Don't go overboard with a VO reverb, try something like the Plate1 reverb, or one of the small room reverbs. Adjust the amount of reverb the voice gets by adjusting the level of the "FX1" fader send. This fader is sending the dry signal to the reverb FX in the bus, while your main track fader is sending the dry signal to your master bus. You basically need to get a good blend of the Reverb sound and the Dry sound, to make it sound natural.

Also, regarding the other suggestions. The Sony preset for "pseudo Stereo" is really not a good choice for creating a pseudo stereo effect. The 10mS delay choice for that preset is too big in my opinion. It should be something like .5mS to .7mS. with the dry undelayed signal panned to one side and the delayed signal panned to the other side. Here's the background on my statements. Sound travels at 1130 feet per second. This comes out to an estimate of 1mS per foot. The average weigth of a persons head is 5 to 7 inches. So to create a pseudo stereo effect you basically want the same signal to reach the persons ears at the sametime from different directions (left speaker/right speaker). So this comes out to a delay setting of .5mS to .7mS as I mentioned. The 10mS setting in the pseudo stereo preset setting is much too large. This is basically a reflection, where it is spaced small enough that your ear can't distinquish two seperate sounds. Sony hasn't consulted me on their presets, so I'll just point out that the name of this one isn't really correct in my opinion for a pseudo stereo FX.

The 50mS suggestion is no good also. This is large enough, where your ear can actually detect 2 seperate sounds. The background on this is that when using a delay your ear can start to distinquish two seperate sounds between 20mS to 30mS. So as you can see the 10mS preset in the simple delay can't be distinquished by your ear as two seperate sounds, and basically makes the sound a little more fuller....that is unless they're phase cancelling, which they probably are and creating a comb filter. So for the 50mS suggestion, you will basically hear a quick echo of your VO, which will really confuse the listener as sounding like a jumbled mess.

A reverb on the other hand has many delays happening at different times, and therefore just makes the sound fuller. The bigger the room size of the delay the futher apart the reflection delay sounds happen. The smaller the room size the closer the delay sound reflections happen. So as I mentioned pick a small room or plate reverb, unless you're creating one of those monster truck commercials.

Firstly a big thanks to Red for some excellent input there, one of the things I grapple with quite a bit. I should have known all that but...
One thing I'd add. Even if you get a mono recording that's in both channels I always select only one channel and have Vegas duplicate it accross both.
Why, well as he pointed out it's all too easy to start getting phase cancellation when it's played back through a mono device. I've actaully worked on a set of 10 CDs that had been 'enhanced' by something that made it sound very odd in mono. Actually it was recordings of the Goons, you'd think the Beeb would have known better.
Also there's some wireless mic gear that has a balanced output on a minipin socket and the odd cable around that wires each side of the balanced pair to each channel, a nice audio disaster waiting to happen.
Bob.

Excellent reply R! - I've often put some plate/reverb to V/O .. . kinda just takes the edge off things . . makes for an "interesting" sound. I've also done the opposite : I've had a repeat of the audio from a "piece to camera" to emphasise a point at the end/concl;usion of a video . . large hall and loud OVER a scene of Gospel singers - worked very well! The audio was a single track from a wireless mic. G