1) Upgrading goautodial 2.1 to goautodial 3.0 is actually an upgrade of Vicidial 2.2 to Vicidial 2.7. Goautodial is actually an "installer" for Vicidial. Vicidial is the "dialer". Although the goautodial distribution does include some pretty screen modifications (and 3.0 also includes a lot more wizards and screens that pull information from the Vicidial database ...), it is still all "vicidial". Please try to keep this in mind.
2) Integration of vTiger 5.1 (AFAIK) will still work even in the latest version of Vicidial Developer trunk (presently 2.8 ). I do not believe any of the previous integration has been deleted or broken, but I've not heard anyone discuss it as "no one" uses vTiger 5.1 (freakin' old! LOL).
3) Customer-data update (as with any prior data exchanges) will require a data-map. SugarCRM (and vTiger) have many more fields and modules for data locations than Vicidial does. Do you want the information in the Leads, Contacts, or Accounts modules? Should the address information be pushed to the Billing address fields or the shipping address fields? Should the phone number go in the work/home/business/office fields (depending on the module chosen, it changes and there are more available in the CRM than in Vicidial)?
That being said, you may want to have a look at Vicibox (another installer for Vicidial without the GoAutodial add-ons or pretty screen, but with all the latest features, some of which are NOT available in GoAutodial ... BUT without the pretty GoAutoDial screens and wizards to help you along). Another note is that Vicibox can Cluster multiple Vicidial servers together allowing for virtually unlimited growth. Kinda handy if you grow large enough. And then you can consider putting your CRM on another server entirely (thus protecting that installation investment in case you need to rebuild the Vicidial server(s) for any reason).
We have provided this level and type of service for many clients over the years in both vTiger and SugarCRM (and a few other CRMs, some of which are purely custom-written).
Once you have your plan of attack, I do not recommend a "get it ALL done at once" approach. I strongly suggest you get pieces done as you go and test them for usefulness.
Also, if you have not yet done so it is a Great idea to install both vTigerCRM and SugarCRM (even if only a hosted freebie or virtual server local install) so you can test how the data is stored and reported and manipulated and see how well either will work for your business model. Find out how each will track your "pipeline" and become useful for the information you'll want to get out of it down the road.
Don't let anyone sell you on either one, check for yourself and read up a bit on how to customize the interface and get reports that you will surely need as you do your job and go looking for information from this system.
Primary concerns: vTigerCRM includes multi-level security (teams ... groups ...: a manager can see the records/accounts of subordinates but NOT the records/accounts of any other managers!). SugarCRM has intentionally held this functionality back. You have three levels: I am an admin and can see EVERYTHING. I am a manager and can see all records in the modules I'm allowed into. I am a "user" and can only see my own records in the modules I am allowed into. No middle management.
Then we have the flipside: SugarCRM code is clean. Their interface is relatively intuitive. I've not bumped into a full release with an actual "bug" in it (aside from things intentionally left out!). I will admit that the latest vTiger interface is much improved, but it comes with ... bugs! LOL. Two releases ago: Email did not work (period). We actually had to put in an external email module to get it to send. Last release: web forms (for importing data) were broken. We had to disable an arcane module that was "deprecated" (and worse yet, it was being replaced by a fresh module that was also deprecated even though it wasn't fully implemented!). Somehow half way through replacement of an old module for a tiny piece of the web form import with a new replacement piece, someone got bored and left it broken. It's a bit of a jumbled mess, so we disabled that piece entirely with no "ill-effect" on the web form import. But the solution in question (reportedly) has the detrimental effect on the "outlook integration module" that causes the tabs to be "NULL" instead of actually having their names on them. Like I said, SugarCRM has cleaner code ... they just leave things out On Purpose (for instance, email in the latest sugarCRM comes with POP3 disabled and it must be enabled in a fairly difficult to find location, unless you are on their "paid" service).
SugarCRM is a commercial package with a "community edition" they would like to talk you out of using in favor of them collecting money. vTiger has no commercial sponsor to pay for code cleaning or make them care about the product, but there is no challenge with chopping off the vTiger logo at your discretion, either.