I've been taking a vitamin c tablet once a day to boost my immune system. is there any benefit to taking it first thing in the morning vs. before bed?

Ideally 2x/day: I know of no advantage of taking vit. C in morning vs. Evening so either time should be fine. However, our body can not store vitamin c and little of what you take in the morning will still be in your body at nite, so for optimal benefits i advise taking at least 250 mg of vit c both morning ; bedtime. See http://lpi.Oregonstate.Edu/infocenter/vitamins/vitaminc/.

No difference: If you don't eat any fruits on a regular basis , vit c can be taken at any time. However studies have not confirmed that high doses of vitamin c boosts your immune system or resistance to colds. On the other hand, vitamin d3 has been linked to improved immune function.