vitamin C is very healing, and would be good to use...but none of us would be able to tell you if something is pore clogging or irritating unless you listed other ingredients in the serum.
Do some google searches for vitamin C serums/moisturizers and use this website to check individual ingredients: http://www.zerozits.com/Articles/acnedetect.htm
You might start with a form of vitamin C that is less irritating than l-ascorbic acid. There is another form that is getting more popular - Magnesium Ascorbyl Phosphate (MAP). Is is less acidic (and less irritating) than L-AA, but still benefits the skin like L-AA. As the poster mentioned above, there is still the issue of making sure you find a Vit C product that doesn't contain ingredients that might worsen acne or irritate skin. Check all the ingredients before you buy anything.
Pure glycerin and fruit fresh. Pure glycerin can be found at most drugstores and fruit fresh can be found at most grocery stores.
I mix these two together until I get cloudy/milky consistency. I then a apply a thin layer to my face for ten mins. Wash off with warm water.
I use this before I do sulfur clay masks. It's really been helping with closed comedones and my other clogged pores.
@ connect16 - If you are searching for an effective vitamin C serum using the L-ascorbic acid, rather than MAP, or other derivatives, then there are a few companies which I've had much success with. Cosmetic Skin Solutions ce ferulic, or their brand name vitamin CE Serum is a low pH formulated serum that brightened, evened my tone, and did an exceptional job in fading my melasma and hyperpigmentation. I also looked at purchasing obagi, but their prices as well as other companies were just too flat out expensive.