I think this is the single most important concept to grasp when talking about panning, and I might add EQ, is creating room in the mix for each individual element. Panning gives a sense of physical space, EQ gives sonic space. I'll diverge on EQ for a moment because it's way important, and even if the OP does understand it after all, others reading may not... The guitar and vocal have a wide range where their frequencies overlap, which tends to cause each to become indistinct, and can contribute to the disappearing/reappearing act the vocals seem to be playing. Volume corrections may not satisfactorily address this. Since you want to get the vocal to stand out, careful listening should reveal the frequencies of overlap, and you can use EQ to cut those frequencies from the guitar. You don't have to be an axe-murderer, relatively small adjustments can yield big benefits.
Back to panning...
This is essentially correct. Nikolai gives a pretty in depth technical explanation of the stereo phenomena, but if you couldn't get thru that, it boils down to there has to be a slight difference between the L & R stereo fields, or the listener will still perceive it as being in the middle, or slightly to one side, as in the 50/25 panning scenario described in the OP. One way of achieving that difference that Nikolai didn't mention is... EQ. In addition to creating space in the guitar tracks for the vocal, try a bass cut on one guitar, and a high cut on the other. Same doubled track, but a much wider stereo field.
I keep mentioning EQ cuts... yes, you can boost freqs as well, but this should usually be avoided, as the sonic results are usually less desirable, and it sometimes makes the EQ interact with compression in unpredictable ways.
I hope this helps.