sugar is vegan but its heavily processed. even the unrefined stuff (known in asian cultures as Gaur or jaggery) is a bit naff as well. Honey can be a good alternative but isnt vegan. Agave nectar has a surprisingly high sugar content, so maple syrup is probably the best unprocessed sugar substitute (in terms of processed, stevia is but its hard to find unadulterated steviol which hasnt been cut with other monosaccharides or dissacharides).
with jam making, I find a bit of an issue with using liquids vs sugar itself is that a massive advantage of using sugar in jam making is that using sugar tends to draw out the fluid from the fruit (i let it macerate overnight), which in turns means that your jam tends to a) have a lower water content (good for baking and shelf life) and b) from personal experience, more pectin is released which means firmer jam. from experiments with honey, this doesnt happen, so am curious to see if maple syrup (proper stuff, not the adulterated stuff you get from most supermarkets) is good for jam making (cant be asked experimenting atm but am interested to see if others try it.
Also whilst on the topic of jam making, you do not need equal portions of fruit and sugar. From personal experience, if you are using berries, you can get away with a 3:1 to 7:3 ratio of fruit:sugar to have great tasting jam with long(ish) shelf life (half a kilo can stay in the fridge for a month in a container, havent seen if it lasts longer as its finished by then). For very sweet fruits, go for 3:1 or lower sugar, whereas a but more sugar is needed for sour stuff.