Both the exams are equally tough.Questions in.BITSAT is not much difficult but the the most difficult part to deal with is timing.You will get very limited timing in BITSAT so your speed and accuracy should be very good whole while attempting the questions. On the other hand VITEEE has moderate level questions and you need good understanding of concepts to solve the questions.
I hope this helps.
Though both the exam have their own exam pattern and type of questions framed for the exam, but I would say clearing BITSAT entrance exam and getting admission to BITS Pilani, is much more difficult as compared to VITEEE.
In BITSAT, questions are more conceptual and level up when comparing with VITEEE. Moreover there are limited seats available in BITS, so competition is higher, which is not the case for VIT.
I hope I could solve your query.