We have the BiltBest casements - you will love the single lock. One thing we ended up doing - our architect had drawn in windows that were 6" larger than we ended up installing. Four rooms have very large windows on 2 or 3 walls to take in lake view. By going down 6", we went from "Mansion Series" size windows to the next lower cost level. Saved a small fortune. He had ALL the windows as functioning - we changed numerous windows to fixed to also cut cost. By doing these two things, we cut 30K from the window bid. Here's a picture of the breakfast area during construction (9ft ceiling):

Hi Ana, I'm still around! Though yes, I don't hang around this forum anymore. So in the time that I made my original post, I finished that old house renovation and then proceeded onto the next project (built a custom home from scratch!). Based on all my experiences, I can safely say that yes, you should absolutely strip off the old shingles/siding before putting up vinyl. For all the reasons stated by others above. In the case of my 100yr old house reno, I ended up NOT doing vinyl at all, instead I found a great Brazilian crew that stripped the old shingles, removed the old tar-paper on the house sheathing and re-wrapped it with Tyvek, and then installed new red cedar shingles. The economy was down then, and they got a deal on a supply of Grade-A real shingles. They were very skilled and did an amazing job matching the original detailed shingle patterns, particularly around the front porch arches - never would get that level of detail out of vinyl. We also replaced all the exterior window & door trim with PVC trim (and re-flashed everything). We definitely went more the restoration route on that project, both outside the house and inside where I re-did all the original natural woodwork. Was a lot of work but so worth it in the end. Also as the original outside shingles had been removed, I had the opportunity to blow-in insulation all around the house (from the outside). On my new build house, I also did shingles. And there again, I realized the importance of installing a good house wrap (in this case I installed one that allows better drainage behind the shingles/siding). You may have already started your project by now, but if not definitely strip the shingles if you can. I'm also happy to give you the contact info for the crew that re-shingled my old house. They don't do vinyl but if you were interested in sticking with wood siding they're great. We ended up painting the new shingles there with a top-shelf Benjamin Moore Aura paint. I've been by the house several times since selling it and the paint is not peeling yet. Here's some pics our house / siding job: 100yr old house renovation

OP, is the siding level above the problem strip? I've only put up vinyl siding once, on a 10' x 12' shed. We found that unless the entire strip was snapped in place, we went out of level. If the top of the strip is nailed level, you should be able to force it to lock.

In response to the above post regarding the rot resistency of wood, pine and fir are very closely similar in this characteristic, with fir being the difference of about a week longer in resistency. If one is looking for a rot resistent wood species, look to cedar, teak, mahogany or cypres, among others. Don't be fooled by industry gimmicks about Douglas fir, or wood dipped in preservatives, etc. If the frame core has rotted on your Andersen window, or any other manufacturer's window, it is time for a replacement. In my opinion, the problem with the Andersen product lies in the use of plastic or vinyl, to clad the structural wood core. Plastic/vinyl materials are disposable products, thus they are not a good protective cover from permanent fixtures of any home. Currently, all wood windows appear to be the best source for long-lasting windows. However, if one is concerned about low-maintenance, the use of extruded aluminum cladding has proven to out-perform roll-formed or vinyl cladding. Contact Andersen and ask them about a discounted price or "recall" on the window with which you are having problems.