If your character has a poor herb lore skill the grains may not be identified beyond something like "interesting grain" (but they can still be planted). However, all agricultural crops can be bought as seeds in villages that grow those crops (you won't have any luck in finding seeds among the northern tribes, as they don't practice agriculture), in particular shortly after harvest (when there may be a small window left for autumn planting), but I haven't seen much in the way of seeds during the summer (presumably all they have left after planting is for their own use).
Also, a small number of wild plants can be harvested and planted. Turnips are very good with a poor agriculture skill as they yield a good harvest (for the skill), while the more difficult crops may have you just get your seeds back (plus some agriculture experience), especially if wildlife starts to munch on your crops.
Turnips is the exception to the rule that the edible part and the seeds are the same thing (well, some wild plants can provide leaves and roots as well), as turnips provide both the root part (the turnips themselves) and separate seeds (and those seeds are edible as well, should you have a need for seeds in cooking and have an excess of seeds).
In general, a completely vegetarian diet is a difficult path to follow, as the nutrition value of vegetables is fairly low.