How sound is
1.e4 e6 2.c4!?, the Steiner Variation in the French Defense, and would you actually recommend it? Does it work well in normal games, or is it more a surprise weapon for blitz?
It's undoubtedly sound in the sense that it does not by force lead to any sort of catastrophic disadvantage for White, but it offers very little if any objective possibility of obtaining a theoretical edge.
If you're specifically trying to reach an IQP position, it's probably preferable to play the Exchange Variation and only then follow up with
c4, since Black has other reasonable options against
c4 on move two besides an immediate
2... c5, for example, can reach positions similar to those found in the Sicilian (i.e., various Maroczy Bind structures that crop up against the Kan, Taimanov, or Accelerated Dragon variations) and English Opening Hedgehog systems. That said, an immediate
d5 seems to be the most popular response, and it's probably what one would typically expect from a French player (and what I would likely play myself).
It's certainly a viable way to play if you enjoy IQP positions and you want to avoid the well-trodden paths of main-line theory. It shouldn't lead to any advantage against correct play, however.
Against 1. e4 e6 2. c4, I'd play 2...e5.
Yes, this moves the e pawn a second time, but White has weakened his black squares and created a "hole" on d4 with the pawn move to c4. It can't be moved back to c3, where it would have formed part of a pawn chain.
If White now plays d4 to repair his pawn position, capture the pawn from e5, then chase the queen with Nc6 when it recaptures.