I'd like to enhance my stomach vacuum routine. What are the alternatives and differences for the stomach vacuum exercise?
I can't speak too much to the stomach vacuum itself, but this is basically an isometric exercise, one where you're working one set of muscles against another, here your abdomen and your lower back muscles. My suspicion is that the benefit of this exercise is less a strengthening or toning of the muscles, and more a matter of teaching yourself to hold that area in, making it easier to hold extra bulk "sucked in". It's not a bad exercise in that it's making you aware of those muscles, and your general core position. However, by itself, I'd argue that it's too isolated.
Instead, if you want something similar, consider the hollow position, a gymnastic exercise where you contract your entire body to bend it forward. As a whole-body movement, you're learning to use everything in concert rather than just your stomach. The link I have above has several variations, but the two key ones are either doing the hollow lying on the ground, or do it hanging from your hands from a ledge or bar.
To experience the hollow position, lay down on your back on the floor. Tuck your pelvis forward with your lower back maintaining contact with the floor. Lift your straight legs slightly off the floor. Extend your arms up straight beside your ears and raise your head and shoulders slightly off the floor. The hollow is an effective body position for the handstand, front lever, back swing on parallel bars, dragon flag, and the bottom position of the wheel.
If you're focused on the stomach vacuum itself, I'd say that your three major variation will be doing the exercise standing (most natural to your natural posture), lying on your back (this one seems to carry the least benefit, but might be reasonable if you're doing this exercise while doing something else, like watching TV), or on all fours (Marjaryasana or "cat pose" in yoga. Doing it like this will be working against gravity and better contrast your movement front and back).