Whether it's vipassana or shamatha depends not on your choice of meditation object (in this case, the space between thoughts) but on how you focus on it.
In vipassana, you closely examine the sensations that are grouped together as "the object" and pick them apart in detail. You're trying to notice the individual moments of sensation that make up "the space between thoughts." This space makes a great vipassana object, but not for beginning meditators.
In shamatha, you calm and tranquilize the mind so that it gradually settles down and stays with the object. You don't examine or investigate the object at all . Instead, you only seek to continuously enjoy whatever is good about that object. The only thing you discover is that it feels nice.
There are 4 ways it can feel nice: an exciting way, a joyful way, a peacefully happy way, and a totally peaceful way. That last way of feeling nice isn't pleasurable anymore -- it's pure peace and calm, with neither happiness nor unhappiness. It's hard to describe how that's nice, but you'll understand once you encounter the fourth jhana.
If you want to use the space between thoughts for shamatha, you could try noticing how peaceful and quiet that space is. See how it has a subtle undercurrent of calm and happiness relative to when your mind is full of churning thoughts. See how that happiness is actually present all the time -- thoughts don't remove it, they just tend to distract you from it. Practice noticing the happiness even when thoughts or other sensations are present, and see how good you can get at this.
If you would like to do vipassana, I suggest starting with a different object. Noting the breath at the nose or abdomen is one good practice, and there are many more. You can read about them in MCTB or at Kenneth Folk Dharma.
