I like to use an old GI Poncho wrapped around my sleeping bag and pad to hold the pad under the bag.  I slip off of it far less and have slept much better since starting this.
Two weeks ago while camping in wet weather, quite a bit of condensation gathered between the poncho and the sleeping bag.  This was inside a tent.
This has happened before, but not to the same extent.  I'm wondering if just using a poncho liner in the same manner will avoid the condensation.
It would be a little bulkier to pack.
Thoughts?

Poncho is not up to the challenge. I suggest GI Goretx bivy. Place your pad inside bivy, bag on top of pad. Enjoy dry sleep.

Probably easier to just use a different pad,  something with a texture,  or put tape on it or something.  Wrapping wiht a vapor barrier isn't advised.

As long as you are in a tent and not using the poncho to keep rain out of the sleeping bag, use the cheapest cotton bed sheet you can find.  

Another option is some kind of elastic straps, or something.  if your bag is big enough could even put the pad inside.

If you're using the poncho to stop sliding around the tent, pick up one of the 1/8 inch thick folding pads from Gossamer Gear

https://www.gossamergear.com/collections/accessories/products/thinlight-foam-pad

It should stop your pad and bag from sliding around. I assume that you're not sliding because of poor site selection that has caused you to set up your tent on an incline

No, I just slip off the mat as I roll over or shift positions.

So you need a wide pad. I had the same issue until I went with a wide pad.

Packing up tomorrow for another trip this weekend.
I have a Gore-Tex bivy and will see if the pad will fit inside.  If so, I will use it one night and my usual method with the poncho the other.
I'm pretty sure the pad itself won't fit in my sleeping bag.
Lowes will be about 35 and we're not expecting condensation.
DirkericPitt describing the poncho as a vapor barrier was pretty descriptive.  I like everything about using it except the condensation.
It would be great if the poncho was breathable with the same size and weight.  I don't ever foresee not taking it as it is handy for a tarp or other things.