You may be able to get away with Vicibox 4.0.3 which installs asterisk 1.4 (and still the latest vicidial code!).
But your challenge is that the ".conf" files (the static ones, that should NEVER be modified) are modified with some new features/additions to Vicidial. Also some are modified during bug fixes. So you would do best to stick with what you have or begin porting your changes to the newer version of Vicidial in a virtual system. Not using the "stock" .conf files will undoubtedly lead to hair pulling as some of the changes almost seem stupid and unnecessary ... until a strange problem crops up that you cannot explain (failing transfers, but only under certain circumstances?LOL).
Note that there are multiple places to put "custom dialplan" and "custom sip settings" into the system now. So you can have them all embedded and never need to customize the conf files again upon completion.
Another method is to put the customizations into an external ".conf" file and just include them in the base ... then your only change to the base is a single include statement. Easily managed after each upgrade.
In the meantime, the best advice I can give you is to fire up a Virtual Vicidial development box and begin your "port" process.
We've upgraded many client boxes over the years, with customizations built by us and also by the client and I've found that some modifications are no longer necessary (as the feature is already in Vicidial!). Recognizing that can save quite a bit of effort. Which you can do after you have a functional virtual system to play with.
Happy Hunting!