But will the vanquish go as deep as the simplex or the at?
Not a simple answer. They're similar enough and there are too many other variables. The tests you see online are often not something you can generalize to your needs.
If you want to skip a bunch of dorky technical stuff, skip to the last sentence.
Soundwaves in music are a good analogy. The long waves of low frequencies (the bass in audio) can travel long distances through all sorts of objects, but aren't as capable of relaying as much detail.
The short waves of high frequencies (the "treble" in my audio analogy) can relay great detail when traveling directly and undisturbed to the listener. For that same reason it loses that detail quickly as it passes through objects and gets absorbed. As far as detecting, that's what the metal junk and/or mineralization does to the higher frequency signals.
Theoretically, a 10" coil can go about 10" deep. But, the waves traveling at the edge of detection are traveling the farthest---through the most trash/mineralization. The short (high) frequencies at the bottom edge of detection are lucky to make it back at all. The lower frequencies can do it, but not with a lot of detail.
Fortunately, silver is highly conductive and US coins are relatively large, uniformly shaped, thick, but often near the edge of the coil's detection pattern. That's OK. There's not a lot of "detail" needed by the detector to get word of a silver quarter and let you now about it. For this reason, many coin shooting detectors utilize a single very low frequency (VLF). 5 to 9 kHz. Entry level detectors with simple (or no) visual ID categories have less of a need for super-accurate IDs. They're also going to be quieter for a beginner or somebody focused on digging some coins a few times a year. Those low frequencies aren't going to report back on every speck of metal in the ground.
So, higher frequency machines are great for IDing shallower objects, and can even perform very well at depth when the ground is fairly clean. People looking for tiny gold nuggets or thin gold chains under a few inches of sand or mineralized ground utilize very high frequency detectors. (Or, choose a different technology altogether -- pulse induction.)
But, there's also a want and need for high detail and depth for medium to deep objects that are not great conductors, such as relics, small gold, and very small, thin silver.
If you were somewhere that has deep coins and very little trash because nobody has set foot on the property for the past 100 years except to mow it, then you'd want a low frequency machine and dig everything that beeped. If the ground mineralization isn't high, then a medium to high frequency machine would even do well.
An all-purpose detector is going to have a frequency in the mid teens. Low enough to get some depth out of an 11 inch coil, but high enough to ID common finds at medium depths.
The AT pro is 15 kHz. The Simplex is 12 kHz. I think the both have a stock coil that's about 11 inches. It seems Notka was leaning a bit more towards depth for coin detection and perhaps is relying on some improvements in post-processing to improve IDs.
Indeed, circuitry isn't the same, and these digital machines process could process the same signals differently based on various assumptions the engineers make about what people want to hear and find.
Processing speed as made multiple frequency machines possible. It's a blend of different frequencies to balance depth, detail, and ID.
The Vanquish is a multi-frequency machine (based on the same technology as the Equinox, although some people suspect it's a blend of 3 frequencies instead of the Equinox's 5.) It's processing a mix of very low to high frequencies.
So, I said all of that, just to say that the Vanquish might have better depth than the AT Pro and Simplex (single mid frequency machines) in some situations, or better ID in some situations, or neither in other situations.