vi.wikisource is currently has the same appearance as the one being used in EN and DE languages, which only contains the phrase "Wikisource". However, the current PNG file is inferior to the one in EN. You can see the two versions here: vi.wikisource and en.wikisource. It is even clearer in 2x version of 2xvi.wikisource and 2xen.wikisource.
Basically, the VI version is less sharp and it has different dimensions (123 x 154 vs. 135 x 155). It results in the VI logo was cropped at the top and bottom when the page is accessed from a mobile browser. I tried to fix with JS and CSS with no success.
I believe that the long-term solution is to change the current Project's logo in $wgLogo to the file used in en.wikisource. It looks like DE project is doing the same thing. Their logo is at https://de.wikisource.org/static/images/project-logos/enwikisource.png which has en prefix.