When I find a vintage glove that needs relacing I try to age the laces to closely match the original as possible. First, you need the correct size lace as far as width and thickness. Some laces take dyes better than others so you need to find a lace in which the tanned side is not as hard and smooth and which feels like it is more absorbent. After the correct size has been chosen, you need to study the original glove laces and try and determine which color dyes or polishes to use.
I use different types of shoe polishes and colors from liquids to waxes depending on the type of new lace material and it's ability to take color. Some of the harder tanned surfaces do not take the liquid color evenly so a wax will be required. It's best to take a sample piece and work with it first to find the right color. I mostly use blonde laces unless I think the darker tanned laces will do. If you use the wax polishes they tend to color better and the edges of the laces round off better also some laces will tend to shrink slightly with the wax polishes. The raw part of the lace on the sides and bottom will always dye darker and become more absorbent.
After you have dyed the lace let it dry this is a must for the liquid dyes, if the lace has absorbed too much of the liquid dyes then you will need to get a dry cloth and run the lace thru the cloth to take out the excess.
After they have dried you will need to check their pliability. If they are too stiff the final application of good ol' petroleum jelly will help loosen them up, but in some case will also darken the lace, so keep that in mind.
Lastly after the relacing process you will need to have a q-tip to apply a small amount of color to the end of the lace after it has been cut. You will find that the laces are easier to tie and the knots will stay tighter.
This is what I do, I'm sure that all the professional restoration people have their own techniques, and could probably offer some better ideas.
For all the people that haven't been doing anything to their laces, please on a vintage glove for display try and make it look as original as possible.
I think you'll be pleased with the results.