So, just to clarify, a VCA will replace the attenuator knob in a Quadratt channel with a CV input, so that one CV can control the amplitude of another signal. This is what you need to apply a volume envelope to an audio signal (the envelope "turning the knob"), but it is also useful for changing modulation signals in various ways. As stated above, some VCAs will amplify a signal beyond its original amplitude, which can be useful (for example, in bringing signals from other synths into Eurorack, which has different standard levels). The recent revision of Veils is currently unavailable (a worldwide chip shortage is affecting the industry and others as well), but fresh supplies are expected in the next few months. There are a number of alternates based on the older version available, notably the Intellijel Quad VCA, or you may get lucky on the used market.
Maths is very popular, but certainly not required, especially if you have Quadrax. You can wait and see if you want it or one of the alternatives. I suspect most users don't demand more from Maths than Quadrax can provide. It's also quite wide.