I'll echo Jim's advice here: don't build this out of a pile of 2-4 hp modules. The result will be a total mess when patching, and then you've got to be able to easily/intuitively get at the controls...which will be a bitch with all of the wires everywhere. Especially if this is an FX device, and your attention is divided between the instrument feeding this and working the Euro build itself. One of those proverbial "recipes for disaster", although not quite that dire...just sufficient to drive you up the wall!
Now, as for VCAs...have a look here: https://www.discogs.com/release/50203-Kraftwerk-Kraftwerk Note the cover art (this is on the British release of both of their first albums) and what that scope pattern is doing.
There's two ways to accomplish that. One is to manually increase an attenuator that's fed by an LFO, but this can have some human "slop". The other is to feed the LFO into a VCA, then feed that VCA with an envelope with a slow rise. Now, consider if that scope pattern was of a pitch modulation signal. What you would have if you fed that to, say, a VCO's FM input, would be a gradually-increasing up and down pitch-waver in the VCO's output. This is why VCAs are important for things that DON'T emit sound...they have the capacity for loads of control signal variation, with this being just a sample.