My advice is to not plant the periwinkle.

Not much grows with perriwinkle, that's the problem. A shade tolerant shrub (rhodo, azalea, spice bush, etc.) that is already a few feet tall could probably survive being transplanted into the conditions you mentioned.

What's the fascination for periwinkle in a raised bed?

Yes I wonder too, why go to all the trouble to create a nice raised bed and then plant periwinkle??? It is extremely adaptable and will grow just about anywhere - shade or sun, moist or dry. It is the only thing growing well under a 50 year old Norway maple with a 36" diameter in my yard!

My experience is that it chokes ALL shrubs and perennials. An established shrub will compete, but it is not flourishing like it could without the Vinca competing for moisture and nutrients. I have had to do lots of weeding to pull back Vinca from other plants in the borders.

Some people say bulbs like Daffodils will grow well intermixed with Vinca, and those do okay, but I prefer to grow bulbs in between herbaceous perennials.

I've got species crocus blooming right now (Z5) which blooms before the vinca (periwinkle) perks up for the summer for a nice look. Well, you can't see them today, since it's snowing -- welcome to spring in upstate NY!

As mentioned above, periwinkle can be invasive, so you are wise to plant it where it cannot spread.
I've found hostas can hold their own against the spread of periwinkle -- so that might give you the large accent plant you'd like. A few astilbes on either side might look nice, too (you mentioned shade conditions).

A shady bed is also a nice place for a collection of native plants -- so that's somthing to think about.

Hope this helps.