A rabbit should eat about 10% of their weight in vegetables (2/3 leafy greens, 1/3 other vegetables), less if you give pellets too (mine eat about 8% of their body weight because they get about 10g of pellets each). Hay should be given at all times, while vegetables / pellets are preferably given twice a day (morning / night). If the amount is correct, the vegetables should be eaten quite quickly, leaving the rabbit with only hay to eat the rest of the time. Giving too much vegetables can indeed decrease the will of the rabbit to eat hay, causing risks of GI stasis, and a lot of vegetables like carrot or parsnip are fattening if given too often or in big quantities (making a rabbit lose weight is really hard, so better be careful from the get go)