What does a tonearm contribute to the sound of a turntable?

Pretty much and if everything is of decent quality it should be fine, don't fret about it.
Now some will say that your losing tons of signal if your leads from the cartridge to the preamp are not one continuous lead but I say BS.
Any decent connection will have such a small loss to be unnoticeable. It takes special test gear to measure the resistance between good connectors, normal test meters won't pick it up.
So sit back and enjoy the music.
BillWojo

 It takes special test gear to measure the resistance between good connectors, normal test meters won't pick it up.

Do we have to go down that well-worn rutted road again?

@fsonicsmith welcome to the club, i have the same Reed 3p Cocobolo "12 :)) Wonderful modern High-End tonearm. I've met people from Reed in Vilnius, Lithuania to get my sample. It's definitely a top quality tonearm. Then i went to vintage route and it's an interesting journey with tonearms like FR-64fx and 64s, Lustere GTS-801, Technics EPA-100 MK2, Ikeda IT-345  

It was you Chakster who caused me to look into Reed. 

@fsonicsmith Wait - I'm curious. What well-worn, rutted road?

@fsonicsmith Wait - I'm curious. What well-worn, rutted road?

With the new Jelco, I now have not one, not two, but six signal interfaces between the cartridge and the speakers:

1. cartridge to headshell/arm
2. headshell/arm to Premiere Interface DIN (little box on the back of the VPI - which I desperately want to get rid of)
   
3. Premier Interface DIN to RCA
4. RCA to phono pre
5. Phono pre to amp
6. Amp to speakers

Is this normal?

Well. let me set you at ease first. You need not worry about 5 and 6 because they are higher level signals. 
It is the low level signal that is most subject to being degraded by connections along the way. Ideally, the cartridge would come with integral wiring from cartridge to phono stage input or to go to the even more absurd (and nearly impossible) the cartridge would be hardwired to the circuitry of the phono stage. 
Connections = convenience. A removable headshell is chief among them. 
Is it worth sweating about? Absolutely not. Only at the Mike Fremer level or the Whats Best Forum level do the connections likely make an audible difference. This is just one man's opinion-my own. 
My "problem" is with a reference to testing. The argument that if it can't be measured it does not count is as old as rocks and persuading the camp that adhere to that argument that they are misguided is like trying to move the Rock of Gibraltar. 
And yes, I agree that the VPI junction box is something to "desperately want to get rid of". I hated how it got in the way of trying to use the conventional anti-skate mechanism. Hell, I hated everything about it. 

@fsonicsmith Cool. Thanks for the clarification. Another thing I've often wondered is how good the tiny wires connecting the cartridge to the tonearm are. With all the fuss and expense about IC's and speaker cables, are these overlooked?