Considering that none of the other Pipboy variants had external vacuum tubes like that, only vacuum tubes behind a grill in the facing as shown here;
http://vignette2.wikia.nocookie.net/fallout/images/c/c2/Fallout_PIP-Boy_2000.jpg/revision/latest?cb=20081206231729
and the new lighting physics would almost beg for an actual flashlight, a designated integrated flashlight in this version would, at least to me, seem more probable.
Additionally, most vacuum tube technologies we've seen where tubes are visible, the vacuum tubes are clear, not opaque as seen here -
http://i.imgur.com/rNFR0iE.jpg
If the "bulb" end was clear with the classical vacuum tube look and amber glow to it, I wouldn't disagree with a probability for vacuum tube, but, since it's opaque, it would stand to better reason that it's the reflection chamber for a flashlight.
As to giving it attribution as a chamber for vacuum tube, Fallout has a long history of showing off vacuum tubes as part of the aesthetic, and it wouldn't make logical aesthetic design sense to have some random tube shape on the pipboy that didn't look as vacuum tubey as possible. Further as far as design aesthetic goes, it can really only be seen 3rd person, where if it was vacuum tube related, it'd likely be visible from the first person view as well, instead of obscured.
If however, it's a flashlight, the flashlight itself doesn't need be seen; only the flashlight lighting physics.
It thus makes more design sense from an aesthetic and "functional" plausibility point that the feature pointed out is indeed a flashlight.