As a photographer, LED lights are, for multiple reasons, the bane of my existence. They flicker, have inconsistent color and intensity output and are generally lacking in the colors that make of skin tones. So I've done some research on them already.
What you are asking about though is the UV output. I don't know what portion of the spectrum vulcanite (rubber) reacts to, but let's assume that it is just UV (non-visible) radiation. So there are two variable to look at: Wavelength and intensity. UV's wavelength range runs 10-400nm. You will have to look up the spectral power distribution curve for the light source you are using and see how much UV is there, but that's difficult because LED manufacturer's curves don't usually cover the UV range, only visible light. Manufacturers claim that LED's don't output UV. True, their output in the 300-400nm range is negligible, but what about the rest of the UV spectrum? We don't know,
but there are suggestions that LED's can output as much UV in the 200-300nm range as they do visible light!
The other variable to keep in mind is light intensity. It is unlikely that any indoor LED lighting in the home is going to come anywhere near the intensity of daylight, so you have that on your side.
In short, I think that incidental LED lighting
might be safer than sunlight for rubber pipe stems, if the intensity is kept low. But there isn't any good data to back that up.
Here's a spectral power distribution curve for daylight vs two different LED light sources. Note the absence of UV data.