I walked the full VF (London to Rome) in one go solo from March to June this year. Your post doesn't mention what month you'll walk this stage but as you intend to cross St Bernard Pass, I'm assuming it'll be the summer months whereas I (&
@timr although a different year) walked very early Spring.
I reached the Martigny-Sembrancher 'dilemma' at the beginning of May. I have a balance disorder (not a fear of heights) which hugely impacts my walking where any kind of path with a drop-off, a narrow path, rock-hopping, tree roots, stream fording, etc is involved. I elected to take the train even knowing Alison Raju's guidebook description was out of date. I was glad I did as I saw winter damage along that section. The guidebook stated no problems from Sembrancher but that was not the case for me. Being so early in the 'walking season', I encountered snow, mud, rockfalls, streams & downed trees. Also the path was narrow with drop-offs in places which made the going very challenging for my circumstances. Although I'll never know for sure, based on what I saw from the train window & experienced after, I feel I would have encountered similar issues/conditions on the Martigny-Sembrancher section.
My suggestion if unsure is make the call at the time depending on when you're walking, the weather, your experience level & knowledge of your own abilities/limitations. If none of the above are issues,
@timr 's advice is always rock solid.
Happy trails!