At times, I feel that the guitar is a cross between the violin and the piano. It is mildly polyphonic, being piano-like. Also, it's notes diminish over time like a piano. Yet it's strings are produce vibrato like a violin, or a horn instrument.

I wouldn't say that it is a cross between the piano and the violin, but I would say that the harp and the guitar are probably the only stringed instruments (or at least the most important) that are capable of being played polyphonically like the piano (music in two parts).

I really want to answer your question but am unsure of what it is...

I'm a professional guitarist, so let me answer this as best I can. I have to say though, Hasntbeen's answer is far superior! The guitar is sort of a cross between vln and pno, yes, in that it is polyphonic and can also produce vibrato. Guitar is strung, but struck, or plucked as we say. The piano is actually considered to be a percussion instrument since it's strings are struck with hammers. Guitar is not mildly , but FULLY polyphonic, as are the lute and other lute and guitar family instruments. I can play for you a fugue in four voices on the guitar. Very appealing thing about it .... The guitar and violin actually do not produce vibrato. The PLAYER does by vibrating his hand/finger mechanism. Violin, of course, is bowed, so that's why the pitches can be sustained and even crescendoed when playing. Struck or hammered strings will eventually decay. Pretty rapidly with guitar since the string length is so short, much longer with pno since the strings are long. You should study the mechanism of a harpsichord. Here is a keyboard instrument that is PLUCKED just like a guitar, not struck with hammers.