You need to determine if it is excess cecals or unformed cecals that you are seeing.
Excess cecals would come out normally formed looking like a blackberry cluster(though they can later get stepped on and squished by the rabbit) and are usually occurring because the rabbit has too rich of a diet(too much protein, nutrients), is distracted(baby bun, in a new home, being frightened by something in the environment), or has a medical issue like dental problems, obesity, or arthritis. When it's a dietary issue, it doesn't necessarily have to do with not getting enough hay but having too much rich food in the diet(pellets, high protein supplements, alfalfa hay, occasionally carbs/sugars/veggies). Feeding alfalfa hay(as opposed to a grass hay) can actually sometimes worsen the issue because alfalfa hay is high in protein and nutrients.
http://wabbitwiki.com/wiki/Cecotropes
If it's excess cecals that's occurring and if you think the trigger is veggies then I would wait til the rabbit is older before trying them again. It's also possible that it could be the alfalfa hay, as that in addition to alfalfa based pellets, makes for a very rich diet and is usually not be necessary to feed both an alfalfa based pellet and alfalfa hay. I usually prefer to feed baby rabbits only slightly limited alfalfa based pellets and free fed grass hay. I don't like using alfalfa hay(except for nursing does, baby rabbits that can't have pellets, or older rabbits that are losing weight) as it's unnecessary in addition to an alfalfa based pellet that is either free fed or close to being free fed. Feeding alfalfa hay can make for a very picky rabbit as well, due to their high preference for wanting to eat it over other foods. When rabbits fully mature and need to be taken off of alfalfa hay, and you want to switch them to a grass hay like timothy, it can be very difficult to make the switch because of how much they prefer the alfalfa. It's just better and much less complicated to feed young rabbits an alfalfa based pellet along with free fed grass hay.
Unformed/semiformed cecals is also called cecal dysbiosis, which is different from excess cecals. Mushy cecotropes or cecal dysbiosis, usually happens because of there being an improper balance of microflora in the rabbits cecum, leading to the cecals not forming properly, and when they come out they come out mushy and don't smell right so the rabbit leaves them and doesn't consume them like they normally would. This is often due to too many sugars/carbs in the diet(pellets, fruit, or treats) and not enough dietary fiber from hay. Other foods like veggies, greens, and sometimes particular types of hay can cause it as well, it's just not as common. There are other possible causes for cecal dysbiosis such as excessive stress, antibiotics, ingesting toxins, kidney/liver problems, and other underlying health problems.
https://rabbit.org/intermittent-soft-cecotropes-in-rabbits/
To correct a dietary cause, usually it requires removing sugary/high carb foods from the diet and may also require reducing pellet amounts. This also results in more hay being consumed which further helps correct the problem. Or if you think it's the veggies that are causing the upset, I would wait til your rabbit is older before trying them again. When veggies/greens are reintroduced, they should be started off one at a time and starting with a very small amount for 1-2 days. If no signs of digestive upset occur(mushy poop, lack of appetite, hunched posture, lethargy, belly pressing, eye squinting, etc) then it's usually safe to start increasing the amount.
http://www.medirabbit.com/EN/GI_diseases/Food/Veg/Veg_en.htm
http://www.medirabbit.com/EN/Food/Food_main.htm
I would also still start to gradually make the switch to a grass hay like timothy, orchard, etc. I would feed enough pellets to last most the day(feeding twice a day) then have free fed grass hay available at all times. As long as the rabbit is eating about a pile of hay the size of it's body per day, then the pellet amount is usually at a good level. It usually ends up being that the pellets last about 8 hours, then the young rabbit eats grass hay for the next 4 hours before being fed pellets again. At least that's what worked best for my baby rabbits, so they got the needed fiber from the hay to keep their digestive systems functioning well, while getting the necessary nutrients for growth.
