1. I wouldn't necessarily assume that it is best to grow Vandas in full sun, no matter which type they are.
Sure some Vandas grow bright, but NOT ALL Vandas grow in bright light. Plus, full sun can be pretty strong depending on where you are in the world. I'd be
extremely cautious about sticking every Vanda you purchase in full sun from the get go.
2. Yes, I agree with "CR7cristiano", please mention the types of Vandas you have. If they are species, please mention the species name if possible. If they are hybrids, please provide the name of the hybrid if you know what they are.
3. How are they being grown? Are they in net pots? Are they in plastic baskets? Are they in wood slat baskets? Are they mounted onto a piece of wood? Are they mounted on a piece of log? Are they mounted onto a tree? Are they in clear plastic pots? Are they in clay pots? Are they in opaque plastic pots? Are they grown with large grade bark as a potting mix? Are they being grown without potting media? Which is it? Different methods of growing present different sets of challenges. It would be of great help to those who are willing to help you to mention what it is the orchid is growing on/in.
I personally cannot grow a Vanda in a basket without any potting media here in the Southern California region. It simply is not possible without giving me a large amount of problems.
4. Fungicide is the least of your concerns, and is probably not going to solve your problem long-term.
You see, what me and "CR7cristiano", and quite possibly what "Vanda Lover" are getting at is that this is a problem with the method of cultivation, not a disease organism problem.
I recommend that the spraying of chemicals, fungicides, and pesticides stop immediately.
Hydrogen peroxide kills cells, that's why it is used in disinfection. This is something that shouldn't be used on a regular basis at all. If used too often, it may leave sores or lesions that are not visible to our naked eye open for infection. Again, please read about the chemicals you are using before applying them.
(Disclaimer: I am not trying to sound condescending - there was a time as recently as 1 - 2 years ago when I myself didn't understand the full ramifications of the chemicals I was using. So please be cautious when dealing with chemicals.)
I don't know what kind of "hydrous oxide" your chemical spray contains, but I wouldn't continually add this chemical compound on anything unless I knew exactly what it was and what it does to certain organisms. The term "hydrous oxide" is apparently a rather nebulous term in the world of chemistry. The company producing this product could be dodging something…what that is I don't know.
Please click on the following link and it will give you some insight on "hydrous oxide":
The Constitution of Colloidal Systems of the Hydrous Oxides. - Chemical Reviews (ACS Publications)
Hopefully, this article will shed some light on what you are likely to be unknowingly spraying on your Vanda's roots. From here you are free to do your own research and find out about "hydrous oxide".
Bottom line is…
Change the way the orchid is grown, and you will get the results you want.
If the way they are grown does not change, you will continue to have the same problems over-and-over again; I can guarantee it.
5. Yes, please do provide photos whenever possible, it will do a world of good to those who are willing to help you.