Even a single patch to something like default kernel config parameters means it's not vanilla. I'm not arguing that there are significant patches, only that nobody seems to ship a true vanilla kernel.
Anybody who cares that much is very likely already compiling their own kernels (I speak for myself here). It doesn't make sense for distros to do the extra work to support it.
Just expanding on this a bit: my Debian laptop has a Kconfig with modules disabled that only includes the exact set of drivers it needs. It takes ten minutes to rebuild on the laptop when I pull from git. Even if Debian did all the work to let me automatically install the latest vanilla kernel... I'd still build it myself.