We will be in Switzerland from 27 August until late September and have planned itinerary for most of it, but having trouble with last 5 nights before heading to Zurich in order to fly home to Australia. I would appreciate some advice from all you experts.

Wengen 7 nights
Vevey 6 nights
Zermatt 1 night
St Moritz 2 nights
Ascona 6 nights

All the above is confirmed, so I don't want any changes. I had planned to spend 5 nights in Luzern but I am now not sure if that will be too long, or if we even need to stay there. Up for all suggestions and prefer not to stay in large city.

Thank you

Well, that's a bit of a mixed message. FWIW:

I'm not sure what your plan is with 1 night in Zermatt -- if the weather doesn't cooperate (and from what I've read, it often doesn't), staying there won't give you much chance to do anything. If your goal is to see the Matterhorn, consider adding time to increase the changes of seeing it.

JMO, but I certainly would not stay in St. Moritz, which I found to be a decaying and rather tawdry ex-resort town now catering to tour groups. Even if your reservation there isn't cancellable, it might be worth paying the cost to skip it.

That said, I love the Engadine, and with 7 nights (your extra 5 + the 2 you had slated for St. Moritz), you could have a delightful time seeing both the upper and lower Engadine, and maybe Mustair, too.

Luzern is lovely. I don't know why you are doubting it.

If you haven't already seen it, you might find some useful information in my trip report:
https://www.fodors.com/community/eur...thanks-983126/

I agree with skipping Zermatt for just one night.
I would use those "extra" nights for either Luzern or Bern. Bern is just beautiful to explore nooks and crannies in the evenings, and for daytripping to Murten, Fribourg or Neuchatel.
Luzern is also a lovely base from which to visit Brienz, Spiez and the Pilatus. You can't go wrong with either!

Ops, looks like I have posted in Europe instead of Switzerland.
My reason for only one night in Zermatt was to overnight before catching the Glacier Express to St Moritz. I am aware the Matterhorn could be clouded in and as we will have spent a week in the mountains in Wengen and surrounds, I thought 1 night might be enough.
Our reason for staying in St. Moritz 2 nights is similar to above...get off the train after a long day, stay the night and look around the next day, then train out to Lugano/Ascona. I could push the dates back in Ascona and stay somewhere in the Engadine area or investigate staying in Bern or stick to original plan of Luzern. More work needs to be done on my part obviously. Thanks

I would probably stay somewhere near, but not in, St Moritz itself.

Luzern itself is interesting enough but I would not spend five nights just doing IT. I notice the trip up to Titlis is not suggested.

6 nights in Vevey? I would hope you'd be doing at least day trips to other locations along the lake such as Montreux and ride up to Rochers de Naye or to Chillon, etc. from there if nothing else but I realize everyone has different favorites in Switzerland, including me. Enjoy your stay.

Day trips from Vevey are easy:
fulll Lake Geneva cruise
Vapeur Park and Aqua Park at Bouveret
Thermal spas at Lavey, Saillon, Ovronnaz, Yverdon, Divonne.....
Fondation Gianadda, car museum, roman stuff at Martigny
Castles at Chillon, Gruyeres, Bulle, Aigle, Oron, Moudon, Grandson, Yverdon, La Sarraz, Morges, Nyon, Prangins, Coppet.....
Lac Emosson cableways
Glacier 3000
Mont Fort
Gorges of Chauderon, Orbe, Aareuse, Trient....
Roman amphitheatre and museum at Avenches
Roman villas at Vallon and Orbe
Watch making museums at La Chaux de Fonds and Le Locle
Lake dwelling museums at Neuchatel and Biel/Bienne
Red Cross museum at Geneva
Olympic museum at Lausanne
etc. etc......
walled medieval city of Murten

Add 1-2 nights to St. Moritz/Upper Engadin IF THE WEATHER IS FINE.

6 nights at Ascona aren't too much if you do some day trips:
along Lake Maggiore (boat, bus , train),
into the many mountain valleys above Locarno
(Centovalli, Onserone Valley, Maggia Valley, Verzasca Valley),
to the 3 medieval castles of Bellinzona,
to Tamaro park,
to the copy of the Last Supper at Capriasca, paint by a disciple of Leonardo da Vinci,
to Lake Lugano (boat from Ascona to Luino, bus to Ponte Tresa, metro to Lugano, boat to Melide Swissminiatur - Morcote etc)

You can easily spend a few days at Lucerne and visit
the Swiss Transport Museum, the Rosengart Collection (Picassos!), the old city and the glacier mills,
Pilatus, Stanserhorn or Rigi,
the baroque monastery of Einsiedeln,
the steepest rope railway of Switzerland up to Stoos SZ,
Berne (just 1 hr by train, OW),
Basel with tons of outstanding museums,
etc. etc.