Not necessarily, you can just call the functions in the C library from D as you'd call them from C or C++ with the added benefit of being able to leverage the D GC, RAII, macros etc.

https://dlang.org/spec/interfaceToC.html

Dunno about the Debian benchmarks game or their build environment. I did my own benchmarks and it was quite easy to write working D code compared to C, C++ or Rust. I used LDC, the LLVM D compliler as opposed to DMD. Dub is not that seamless compared to Cargo but given that you have to set things up manually, it doesn't encourage dependency hell.

If you're writing networking code, Go is probably a better choice than vibe.d.