what should i do after pruning if i keep it outdoors in winter? what do i cover it with?
Mulching over the roots and protecting the top portion will help your plant survive the winter. You can cover the top with a tent of burlap, plastic, or other fabric.
https://www.gardeningknowhow.com/garden-how-to/mulch/mulching-plants-in-winter.htm
If you live in a colder climate, see this article:
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/winterizing-mandevillas.htm
What does my plant need feeding ? I has flowered brilliantly since planting 6 or 7 years ago until now
This appears to be a nitrogen deficiency. Alternatively, it could be that your pH of the soil is too high. Iron sulfate will bring it down, if results show this to be the case. Otherwise, You can feed with something a little nigher in nitrogen. An all purpose won't hurt it. Here is an article that will help: https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm
I try to get seed of Mandevilla vine but not yet, Please help me in this regards.
If you are not getting seed pods, it might be that the proper pollinator is not present (certain butterflies it seems). Hand pollination might work, but I'm not sure.
If the flowers are pollinated, seedpods should appear after the flowers fade, which gradually trun brown and split open. These contain the seeds. This article will explain the process:
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-propagation.htm
brought mandevilla inside for about 3 months now. it has bugs (yellow ones) I have sprayed but no luck. should I cut it back and let it go dormant now?
Mandevilla is sometimes attacked by yellow Oleander Aphids. Please see this article to see if the photograph is a match:
https://www.gardeningknowhow.com/ornamental/shrubs/oleander/get-rid-of-oleander-aphids.htm
These articles list some additional control methods:
https://www.gardeningknowhow.com/plant-problems/pests/insects/homemade-aphid-control.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/homemade-soap-spray.htm
With the mandevella plant in an outdoor pot, what action should be taken if quite a few leaves turn yellow? please note in summer.
There could be quite a few reasons why this is happening. This article will help you narrow down what is the most likely issue: https://www.gardeningknowhow.com/ornamental/vines/mandevilla/yellow-leaves-on-mandevilla.htm
I brought the plant indoors,it sits in a south facing window, gets plenty of sunlight. I'm getting new growth along with leaves dropping. Should I trim it back or trim the branches with dead leaves. i live southern Delaware are winters can be very cold.
Mandevilla goes dormant over the winter which is probably why it is dropping leaves. Ideally, it should be in 60-65 F room and watered sparingly. Come spring and brighter light (winter light does not simulate summer light), new growth will appear. It is then time to restart regular fertilization and regular watering.
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/growing-mandevilla-indoors.htm
Mandivilla growing northern climates.
You will, likely have to grow these indoors. This article will help you in doing this:
https://www.gardeningknowhow.com/ornamental/vines/mandevilla/growing-mandevilla-indoors.htm