The upload buttons on the scene form send videos straight to your Dropbox in chunks, so multi-gigabyte masters are fine.
Before it starts, Vaultmastr checks your Dropbox has room for the file. If it doesn't, it says so straight away instead of failing at the end — free up some space or move to a bigger Dropbox plan (Setup → Cloud storage compares them).
While a file uploads, a progress card shows the percentage, how much has gone, your upload speed, how long it's been going and roughly how long is left. Thumbnails, photos and documents show the same card, then a short "saving" step while they're resized and filed in your cloud.
Keep the tab open until the card says it's done.
If your connection drops, Vaultmastr retries on its own. If it still fails, choose the same file again in the same slot — it carries on from where it stopped instead of starting over (for up to 40 hours).
You can also put big files straight into the scene's Dropbox folder — Masters for the full scene, Trailers, Thumbnails — and Vaultmastr attaches them when you open or save the scene.