Have you considered staghorn sumac. It prevents erosion very effectively. It is invasive, but I merely cut back the saplings that escape from their designated area once or twice a year. I love the colour in the fall and the birds love the "flowers".

There are two perennials that come to mind. Thrift and Tickseed. These two thrive on slopes and are great for stopping erosion. They are hardy in your zone as well. You could interplant your daffodils among them and it wouldn't hurt a thing. Haven't visited NY in over a year, I certainly love driving through the upper state.
Good luck and stay safe

unfortunately, the two best erosion controlers are crown vetch, and common hemp- the one's become a nuisance, and the latter's well, illegal to grow in this country (stupid people)

the day lillies are the standard around here for a better choice- they're commonly known as 'ditch lillies' around here, in fact.

rather than a retaining walll, a simple line of small boulders as the base of the hill should provide SOME support until the plants establish themselves...

Check out buffalo grass or other native grasses. They do not need frequent mowing. If you are willing to kill off the turf grass (doesn't sound like it's holding the soil anyway) and reseed with something that doesn't require mowing, that might solve the problem. Add some flowers, too, and you may create something you can enjoy instead of a hazard that might kill you or someone else.

how about groundcover junipers? they are evergreen and low maintenance.
native grasses is a very good idea, too.

Perhaps the problem could be solved by finding a complementary plant that could be sown to replace the grass. Perhaps you could do this with a selective herbicide that would kill the grass, but not the existing vines. Possibilities that come to my mind might include tall ornamental grasses, various wildflowers which would grow in a similar environment. Mulching heavily among the vines might help retard erosion and give seeds protection while they germinate, and increase moisture to allow better establishment of plants. Disturbing the soil much in such a locations seems like something to be avoided. Perhaps a "short" terrace system would be useful, consisting of a single row of railroad ties anchored with 4' rebar driven through holes driven through the tie into the ground. This serves to slow runoff, and help plants establish. Diverting waste water to this location from your washing machine might encourage plant growth.
I have seen a stiff fabric "honeycomb" used by a local park to control erosion. The openings are about 8 inches in diameter, and I think they are about 6 or 8 inches deep. The fabric is laid out, and filled with topsoil. This seems to work very well, but it is probably an expensive solution.

There is a product called biologs that can be staked like the honeycomb mats that sqlguy was talking about. They're both hemp, and return to the soil as the plants establish, and with the biologs you can slice into it with a knife and plant forbs or grasses directly into them; the fibrous nature of them both holds water and allows gas exchange equally and is organic, so about as good a rooting medium as you could ask for... Here's a source

Here is a link that might be useful: Erosion control products

Rhus aromatica 'Gro-lo' is a suberb low growing shrub for maintaining steep banks. About 2 feet high, with great fall color. I recommend it to all my clients who are talking about Crown vetch (horrors!!!).

My wife and I were thinking about using Crown Vetch for the side of a hill in front of our house. The hill is the length of our house, with some of it virtually impossible to mow. What is wrong with Crown Vetch? I tried finding the 'Gro-lo" but can't find it anywhere on the web. Any suggestions? There is currently grass covering the hill. What do I need to do to change it over to ground cover?

Steve

Rhus aromatica is eaten by deer, right? can anyone recommend any deer resistant ground covers for shady conditions & erosion control (i'm picky, i realize!)?

I had this same bank at my old house. I tried daylilies, but they don't multiply fast enough and when you do go in and weedwhack you whack the lilies because the look like grass. Bishop's Weed (Aegopodium) was the solution for me. It actually likes to be mowed now and then and it will quickly fill in and shade out most of the weeds. This stuff is invasive, but for something like this it worked wonders.