Write a cover letter with the explanation you just gave regarding your business and self-employed status, and attach 1-2 sample contracts and perhaps invoices. If concerned about confidentiality, can redact client names. Also consider attaching business registration document as this seems to be asked for. Compile all as a single pdf to submit as only one document can be uploaded per question (at least during initial application).
each question allows one document to be uploaded, so if you want to submit multiple documents (for a question) you'll need to merge them into one, or submit one and wait for the embassy to request further documents
Upload the biodata page of passport where it asks for that. Upload location documents (including visa stamp in passport) where it asks for proof of location.
yes HCMC embassy seems to want both the stamp and hotel confirmation/receipt. It's too bad multiple documents aren't able to be uploaded. So you have two options. (1) Submit one document, apply, and they'll likely request further documents. (2) Create a single document (e.g. merge PDF) which includes both/multiple documents you want to submit for the location question.