From what i gather from other posts on this subject the problem with new tubing is that the id is smaller than oem tubing of the 60's. This is true , i've seen the difference. The new stuff cracks after some hours of running the engine. I'm going to try laytex tubing. It's much more stretchable than rubber. The difference in the od of the fitting and the id of the tubing can be taken up by shrink wrap tubing. That way you can buy 1 or 2 sizes and jury rig the size with the shrink wrap.
The only problem is color coding. There is colored tape to acomplish this, i think.
I emailed a manufacturer about the heat environment in the engine compartment and how the laytex would stand up to it.
USA Parts sells the color coded vacuum tubing, and while it is not factory, and only available in the small size, it resembles original better than nothing. I have used the standard NAPA name brand vacuum tubing in the various sizes and it works fine and holds up. Don't use fuel tubing, that does not hold up as well, because it is not really designed to get too hot. It seems much more rugged, but heat is the rubber fuel tube's kryptonite.
Vacuum tubing and devices are vital in these mid 1960-1980's Cadillacs, and have never had a problem using these methods.
You also have to make sure what you are using is rated for vacuum use. Too flexible and too thin a wall it could just collapse especially where its going around bends. Likely why a lot of the factory stuff went to hard(ish) plastic in the 80's.
TJ I tried NAPA's tubing. to me the id was smaller than the original. About a week later the tubing cracked at the fitting.
I had not thought about the vacuum pressure.