All these comments are true, I'm sure. As long as you have Rodinal on your shelf, you may as well try adding the ascorbate. I found that the character of Rodinal is still there, but with finer grain and somewhat higher film speed.
If you get Vitamin C from druggist or heath food store, be sure it is not dehydroascorbic acid or ester C. Dehydroascorbic acid is converted to ascorbic acid by our bodies, but not by our films. Ester C is usually a calcium ascorbate or something similar that is not very soluble in water. Oil doesn't work very well as a solvent for developers. If you get ascorbic acid or its mirror image erythorbic acid, be sure to neutralize it as previously mentioned with sodium bicarbonate or sodium hydroxide or sodium carbonate. You could also use the potassium equivalents. Without this neutralization, the pH becomes too low for development. The bicarbonate you buy at the grocery store is quite pure, and if you are squeamish you can weigh it and the ascorbic acid to get equimolar amounts. The ratio will be 40 parts of NaHCO3 to 176 parts of ascorbic or erythorbic acid.