How to get a vintage/dreamy look like this photo?
It has some yellow color cast and seems like extra blur added in post process?
There are a number of factors here, two of which you've identified:
These are important, but part of the dream-like appearance comes from the high key. Here's the histogram for the image:
which shows that all of the tones in this image are brighter than the 50% mark, with the bulk being way up in the top 20%. Nothing is even close to dark, let alone black. This brightness, especially when combined with blur or soft focus glow (not really seen in this example), tends to contribute towards a dreamy, ethereal appearance.
If we use the Levels tool to bring up the black point, stretching that same histogram across the whole range, the image looks like this:
and if we use Auto Levels to correct for the yellow cast in addition to that stretching, we get:
Both still have the blur, but are much less dream-like in appearance.
This is kind of extreme, and the high contrast now looks like an effect in its own right. The "realistic" image is somewhere in the middle.
I think it's pretty clear in looking at the adjusted images that the blur is from a shallow depth of field; the flowers are in narrow band of focus and the nearer and farther objects are blurred. This suggests that a long focal length was used, in combination with a wide aperture. It doesn't seem like post-processing blur was added, although it's possible that there's a smidge of it in the bottom front.
This also brings out what looks like corner vignetting, but that may just be the framing of foreground objects rather than either lens obstruction or a post-processing effect.
There is something called a "soft focus lens". In terms of getting a dreamy defocussed look, I would say reducing "clarity" in post processing woudl achieve it- or some mild blue applied to the image.
However in this case, I just think it is a longish wide aperture lens which blurs the foreground and background while having a strip (the flowers) in focus.
Vignetting is amplified by lenses when they are "wide open" or can be added in post processing. I would be inclined to think that the vignetting exhibited in the image stems from the lens.
I see significantly dropped contrast, boosted brightness, slight desaturation (perhaps some light selective desaturation as well) with a very narrow depth of field. I think the yellow cast may come from the desaturation and brightness boost from what may have been a brownish yellow ground cover. (It could also be yellow cast grass, but the flowers themselves don't seem to be so yellow cast. It could also be a green desaturation.) Vignetting on the edges also adds to the feel.
For reproducing the look, the desaturation and lack of shadows (making it feel washed out) as well as the vignetting is key to giving it the slightly unnatural but recognizable look that feels kind of like it is in the clouds/dreamy.