I recently swapped nibs with another FPN user. He had a Vanishing Point with a gold nib. I had a Decimo with the silver colored nib (which I thought was gold, just Rhodium plated).

The gold VP nib unit seems to be a hair too fat and doesn't slide up and down smoothly with the push button.
I've tried the gold VP nib in two different Decimos, so it's not a one-time manufacturing tolerance problem on the pens.

On the nib unit there's a small nipple that fits into a slot in the nib end of the pen to ensure proper orientation. As soon as that nipple is about to disappear (like when the button is pushed), I can feel resistance. When I unscrew the pen, the nib unit doesn't slide out with gravity alone, but I can pull it out.

Any suggestions?

Dillo can probably give a better reply for this as he can actually carry out the modifications to allow the VP nib to fit in the Decimo.

I believe the nibs are practially identical (and maybe the Rh nib is the same in both Decimo and VP) but some of the gold VP nibs have those protuding parts that are a little too big for the Decimo and need to be filed to ensure a smooth fit.

Hmmm, thanks for this. According to Richard Binder's site they are interchangeable and I was just about to order one for my Decimo. I'll now specifically check with him before doing so.

I bought another nib unit from FPH and swapped out easily into my Decimo, so maybe it depends on manufacture date, etc?

Current-production 18K VP nibs will fit the décimo and Fermo.

Older 14K nibs won't: the key that orients the nib unit in the barrel protrudes too far from the surface of the nib unit. We have a few of these older nibs mixed in our stock, and I'm careful to modify them before we ship them.