V sits and Hollow

When performing v ups would it be considered useful to keep the hollow position during a complete set of v ups and never letting legs touch the floor? V ups seem too easy now and I'm thinking of making it a but  more challenging

That is one way to make them more difficult. 

 Yes, that is the proper way to do them.

 

 Another would be to do them with ankle weights or a medicine ball or something like that.

 

 But's honestly not something I focus on for strength besides V-up drills like V-up to V-sit.

Once the repetitions start to get very high an exercise can become less interesting.  The V-up may still be useful to you warming up or after you are fatigued but I would move on to training a more demanding exercise, there are so many to choose from - hanging leg lifts, ab wheel, v-sits, etc,etc. 

The V-up is an interesting exercise to gauge your progress because people in the general public think it is torture and for gymnasts it becomes easy.

 V-ups for the masses, in the fitness world, tend to be done in high reps from what I've seen. Also bare in mind, V-ups and hollow stuff are much more difficult for many adults having to move and manipulate bigger bodies (length and body weight) than some 4 foot midget who weighs 55lbs.

 

 Typically in gym, we use them for conditioning though some programs use the V-up press to momentary L/V for strength or just something different.

 

 One of the fun things to do for conditioning is say 3-5 V-ups, roll to stomach, 10 arch rocks, roll to hollow repeat on both sides for 3 rounds before resting. Or other combinations such as V-ups roll through superman to hollow and more V-ups.

 

 Err maybe not "fun" per say, actually. Fun to give gymnasts. They suck because of the TUT.

V ups should always be in your warm ups, because it is an efficient way to maintain the active pike compression that you need to build over time. You can't just do these for 3-6 months, master them, never visit them again, and magically expect to have great pike compression forever.

 

Maintaining these, assuming you are doing them properly and are getting pretty good compression, will make the learning curve for HLL much, much shorter.