The vib/trem on these Vox amps is a real weirdo. I am assuming that your AC15-clone circuit is the 1960 era one, with pretty much the same as the AC30. This one has the vib/trem switch and uses an ECC82 and an ECC83. The earlier design uses the pentode section of an ECF82 in the "Vibrovox" unit, which only gives tremolo, or amplitude modulation. Vibrato is frequency modulation and is much trickier to achieve. The circuit that Dick Denney used (pinched from Wurlitzer organs) uses a myriad of valves, as you have found. I have done quite a lot of searching about this and come up with some interesting stuff. I could probably give you a few leads, if you are interested. I notice that you have a normal, panel-mount potentiometer for the preset depth control. You would be better off putting a skeleton-type in there, as a few bumps could cause the pot to come adrift - guitar amps do get banged about a lot! This control set at mid-range gives a reasonable tremolo level, but barely any vibrato. You are right, it is something of a "warbling" effect, but should be relatively slow. It is somewhat akin to the use of a whammy-bar on a guitar. There is a good case for not incorporating any kind of tremolo circuit when building a clone of a classic amp. It is rarely used these days and would tend to be switched off, if existing. There are exceptions to this rule, of course!
I have looked more closely at an AC10 chassis photo and have revised my thoughts on its length. It only has four input jacks, which are quite close (probably as close as on the AC30 with integral top boost), five chicken-head control knobs and the mains controls look to be closer than on the AC30, too. So, I'm guessing at about 16" now. The reason for the bottom part of the chassis in the Contempo being steel is that all the heavy stuff (transformers, choke) are mounted there and aluminium would probably sag under the weight (the earliest Marshall amps were clones of the Fender Bassman, except that they used an aluminium chassis, rather than a steel one. Some people claim that that is a reason why JTM45s sound different to Bassmans).
OK, confession time...Some years ago I stupidly changed all of the small components in my AC30 for modern ones, including replacing carbon-comp resistors with metal-film ones. The sound was a somewhat "cleaner", but I had not only sacrificed the original sound, but I had also reduced the value of the amp significantly. I wasn't so stupid as to throw away all the replaced components, so what I am doing now is to put it all back to what it ought to be, except for replacing out-of-tolerance, faulty or broken parts. OK, so if you are making a clone of a classic, using whatever resistors are to hand probably doesn't matter, so long as you are happy with the sound. I am also making a clone of a Fender Deluxe Reverb (with some mods) and I chose to use carbon-film resistors, rather than searching out carbon comps. Some boutique amp makers favour carbon-film resistors, so I though that was a good argument. Carbon-comps can drift with age, but if the amp is a classic and is working, it may be that the sound you have, while not like when it came out of the factory, is actually what you love. This argument has been put forward by Dan Torres in his book "Inside Tube Amps". All three of the books that I have mentioned are full of excellent information, but I can recommend you get a copy of Stephen Grosvenor's book. You can get it from
http://www.thevoxac30guide.com/ and at £UK9.99 it is a steal. Jim Elyea's book is rather pricey and shipping to Australia would probably be prohibitive. The copy I have is the Deluxe version with the schematics portfolio. Some of those circuit diagrams are the very devil to locate anywhere else.
Bye-the-bye, the classic "chicken-head" knobs are not all the same. Some have a wider pointer end. The ones used on Vox amps have a relatively narrow pointer.
Caveat emptor!
As it happens, when I make a clone amp, I try to make up a logo that is similar to a manufacturer's that I think the amp is similar to. So, I have an "Earp" in ice-cream script because it is "some sort of Marshal(l)", a "Mega/Bluesy" which is inspired by Mesa/Boogie and my Fender Deluxe Reverb was going to have a Fenderesque "Fooldya" logo until I realised that the Deluxe Reverb didn't have a Fender logo.
If you want any more help, just ask.
Good Luck, Colin.