I do lots of pen blanks dying, using all sorts of dyes but, if used in conjunction with the Cactus juice, I use Alumilite dyes.
Yes, vacuum is better than pressure when it comes to soaking through a dye or any liquid however, the only reason why I have to dye full size pen blanks, is because they are made for sale and people expect an full size blank with full colour penetration BUT, if I'm doing a pen that I want to add a dye colour and the wood is stable enough to not stabilise, I don't waste my time and money (extra dye and solution to dilute it with, wood, electricity, etc...!) and the possibility that the colour didn't penetrated deep enough and were its needed, after all, from the full mass/volume of a full pen blanks, only about 10% is actually used on the pen barrel, the rest is just waste so, I prefer to dye the wood when the barrel is turned to size.
Anything can be used as a dye, from water to oil based to anything else with colour, will do the job, including any of the colour markers most kids use at school.
There is no need to soak the pen barrel, that can compromise the gluing of the tube strength so, only you need is a piece if soft rag, a strong paper towel, etc., to spread the dye over the wood surface, if using a fine point marker, remove the cartridge from the inside and use it to colour the wood while the lathe is spinning.
You would be surprised how good of a job, shoe polish is for this purpose, in fact, to colour and protect the wood, just think, what shoe polish is made to do.
The colour only needs to be on the wood surface so, if using any liquid dyes, put a small portion in the soft applicator and rub it firmly to the barrel, if the colour is uniform and has the depth you want, use a clean soft applicator to "burnish" the dye, there is use the friction and heat produced to burn the dye into the wood and dry the wood surface at the same time.
I you use water based dyes, make sure to let it dry a bit longer, 1 hour or so, if the shop is not too cold and you can put a warm light next to it.
When dry, use fine steel wood to smooth the wood, sanding paper can be just too much and remove some of the dye making it look "spotty" and you don't want that so, rub it smooth and then coat it with CA or any of your preferred finishes...!
Hope this helps...!
Good luck
Cheers
George