As far as the flex of this tubing, I can't say I've ridden the Vitus 980 tubing on a Peugeot.
But on my 57cm Limongi (full 580) I can't notice any excess of flex, and the bike is most responsive to any quick, spirited transition to the standing position.
I'm just under 150lbs fwiw, but do ride hard in the hills here.

The geometry chart for the Peugeots of this later era shows an unusual "step function" occurring in the middle of the size range, with the 57cm and larger frames getting a much-steeper 73.5cm headtube angle versus a very shallow 72-degrees on the smaller sizes!
So, just as with the huge "chronologic" geometry shift that occurred between 1972 and 1973 on the PX10 model, again these later model's geometry and handling can't be described at all without stating what frame size we're talking about. In either case, it's likely to create disagreement when describing any of these bikes by model (without including year or frame size).

What I can say though is that many bikes can show either twitchy or stable handling, based on what stem length extension is used, and the resulting weight distribution over the front wheel!
Note that said weight distribution can be severely affected by a rider's tendency to "hang back" while descending at high speed, the worst thing that one can do if there is any tendency toward shimmy or instability.
And yet there is some tendency for many riders to do this, "as if" being fully forward might threaten to put them over the bars(?).
Staying put in a "charging ahead" position in the drops almost guarantees maximum stability (assuming here no extremes in terms of stem extension length).