I have narrow cast iron brackets that can be window box brackets or can hang off the cast iron rail on the doorstep. I have mine on the doorstep. A rectangular planter slides into them and I remove it in the winter. There's no way they would catch snow or ice, but I would make sure that they were securely screwed into something solid behind the vinyl. I have NO idea where they came from as they came with my previous house.

Thank you. The snow slides off my house and presses into my sides of the house so that is what I meant when I mentioned they would catch the snow and ice... It comes back into my windows too, so it causes me concern every year for that reason. I thought that brackets would work, smaller and maybe less in the way.

I have 4 window boxes in front for over 15 years, I do take them down
after several frost in fall....they eventually need to come down anyway
for planting again come spring. What I did when putting up, they came
with heavy steel wire brackets, I made a piece of fir wood and grooved
the back where the brackets would fit in...they can be taken off, just slide out.
The wood I painted the same color as the siding. [ I have wood siding] I'm sure,
you can put over vinyl too....just make sure the piece of wood is screwed
securely into the wood below the siding with a bit of marine goop or
silicone.