Core - 100 Days to Abs

I've been lifting and strictly watching macros since October. I've seen significant changes in my body - more toned arms and shoulders; firmer legs and larger glutes; abs peeking through at the top and a solid line down my stomach. I have a very regimented upper body work out, as well as a lower body work out that I stick to successfully with lots of results. I lack when it comes to core exercises. My main routines include the Roman chair (30 reps, 4 sets) and side incline reach with a 25 lb. plate which targets the obliques. I always want to do some exercise ball circuits (crunch with 25 lb. plate, planking knee-ups, twist with 25 lb. plate, etc.) but lately I haven't had time. I need to start ensuring that I have enough time in my upper body routine to get every exercise done to target both arms/back/shoulders and core. I have the basis, but there is of course, still a layer of fat covering the "ripped abs" that I'm sure are hiding underneath. My question is, what can I focus on in the next 100 days (I have a vacation that I'm going on in 100 days that has been my "motivation," although it has now become a lifestyle...) to make sure that my abs are toned while I'm in Croatia. Exercises are much appreciated! I know a lot of core strength comes from stabilization in other workouts, but I'd love to target those muscles even more than I already do. Also, how much of this is diet and how much is strength training with a core focus? I eat about 1,400-1,600 calories a day (moderately eat back exercise cals) and strength train daily. High protein, low sugar diet. Just wondering if I should be attempting to "cut" when the event gets closer, or if I should be maintaining a diet and increasing the core work outs. Advice needed and appreciated.