do you mean sforce.one.createRecord(entityName, recordTypeId)? If so, I'm not quite sure if it is fully working in Spring '14 as the last news that I had were that It was going to be ready on Summer '14.(Last time I tested it, but going to test it again! :) ) However the sooner the better.
But I think to your main question is about how to implement VF pages in SF1 and populate values.
The idea is that the VF pages are going to continue the same way as they were, but with the option to be visible in mobile directly clicking a checkbox.
Then in order to pre-populate fields you will need also a VF page in opportunity, and your controller will send the values to populate fields. Then here is your case: you have your initial VF page that will send you to Opportunitiy ( might be using navigateToSObject? or createRecord) and in opportunity you will have another VF in order to pre-populate fields and after everything is correct click save and create the new opportunity record, unless you created already your opportunity from your initial page and navitate to it using navigateToSobject ( I personally like this one and I used it already). Then in this case here you wont need VF page in opportunity, as the new record has been created in your initial VF-controller and you are only navigating to it and showing it.
Now comming back to how implement VF for SF1, I personally have in count user experience and where is going to be use the new VF page: only mobile? in desktop and mobile? I try to give same experience in both and be consistent. Might be use a little of JQuery? But appart from that it should have to be any difference to build a VF page for mobile than for desktop. Salesforce allows us to build the VF and with a simple click on Available for Salesforce mobile apps.
There are also some known bugs around:known issues
Apart from that the best guide that you can use in order to build VF pages is this one: VF Developer guide
Ah! I just forgot 2 really nice links from Keir Bowden and Sandeep Bhanot: Salesforce1 for Visualforce Developers / Salesforce1 and Visualforce - Things I've Learned
and the other one Using Visualforce in Salesforce1
Hope it helps! If I can help further let me know :) - and sorry for the long answer -