> As sort of a vacuum in spacetime?

Definitely not. For one thing, gravity isn't just mass. The gravity of a moving baseball is higher than a nonmoving baseball. The gravity of a charged battery is higher than a discharged battery. Same protons, neutrons, and electrons, but if you change their velocity or how they are arranged then they have more gravity. It's the stress-energy tensor, not just the mass (by which you mean matter?).

> Could gravity be the effect of mass in an indefinite form?

A very clear way to see this is not true is to compare stars and black holes to quarks. The space between quarks acts like they are different, larger or smaller quarks depending on how the real quarks happen to be arranged at that moment.

So, if we apply the same idea to a very large star, it would sometimes act like a black hole or a supernova or a star depending on where it was and how it happened to be doing and also maybe just sometimes spontaneously?

But mostly, if it was really far away you would expect it to act like a black hole, because from a far distance it seems like its all in one small point, and a black hole is an irreversible phase transition. But that doesn't happen, and stars look like stars no matter how far away you are. If reality worked like this, we'd expect nearby galaxies to have almost no dark matter and faraway galaxies to have lots and be very small. But instead galaxies and dark matter are pretty much the same no matter how far away they are from us.

This is a pretty ill-posed question though. I'm cramming it into a box that has an interesting answer just for fun, and I don't think it answers much of what you are asking.

We see gravity whenever we see large "clumps" of matter. We keep looking for a physical property of matter than explains gravity, but perhaps it's the lack of something. So when matter appears to attract other matter, maybe they are both "falling towards something".

> We keep looking for a physical property of matter than explains gravity

No, the Einstein field equations actually wrap things up pretty neatly as far as why it happens (although not how). Energy, momentum, stress, and mass are all the same. Energy turns into matter and vice versa. E=mc^2.

Going faster doesn't change your velocity relative to light, just your reference frame. In the same way, getting heavier changes your reference frame. If you're changing your relative time, you're also changing how far you're traveling from an external reference. And if the distance is changing, it means space is warping... which is exactly what gravity is. Energy/mass/momentum warps space and that gives us gravity.

I always think 'an experiment' could be said better, because the thing doing the measuring is also a haze of probabilities and the measurement is how these two probabilities interact and change each other.