Regardless of whether your managed package has Lightning Components in it or not, if My Domain isn't enabled, the Visualforce you currently have will display.
If your package doesn't have any Lightning Components in it and My Domain gets enabled along with Lightning, your Visualforce will display normally as expected. Salesforce will automatically open a new tab to display your Visualforce unless they're embedded in a Lightning Page in which case a new tab won't be opened. Instead, your Visualforce would need to be wrapped inside the appropriate code.
If the latter applies, see the Trailhead Modules for more on using VisualForce within Lightning pages. I'd also recommend you visit the Partner Community Lightning Readiness Forum as well as their Lightning and Component Forum if you haven't already been there.
I'll add that be default, My Domain isn't enabled in a Trialforce Org. That's something a customer would need to enable themselves during set-up.
I suggest you go through the Trailhead Lightning Components module. I think it will become clear that your components will need to be namespaced with your domain in order for you and others to use them. It's what distinguishes your Lightning components from other components and how they're referenced. All orgs will need to enable My Domain to use the Lightning Experience.
There's also a new module I suggest you go through titled Lightning Experience Development that may help answer some of your additional questions. It includes a section titled "ISVs, Packaging, and AppExchange". I suggest you try to remember that Lightning isn't yet a fully developed platform and that what's published today could change tomorrow. Salesforce itself admits it's something of a beta that's in progress. There's much that's still to come as Salesforce continues to fill in gaps in the Lightning platform that haven't yet been addressed.