It's a nice piece, but I think one of the biggest missed opportunities with Arnold was to make several different renditions of it throughout the CR. What makes "We Have All the Time in the World" amazing is how Barry used it in OHMSS throughout the film in different sets of moods. There's a laid back jazz orchestration for when Bond and Tracy are at the casino, then we would get a whimsical take when Bond is being escorted by Draco's men, followed by Draco telling his history to Bond. There's then the swelling romantic cue between Bond and Tracy at the barn. Finally the movie ends with the tune being played on a very tragic note, just a simple flute solo and strings to really hit it hard.
Arnold never really goes for that in CR, and because of that I think it gives away too much. The tune is always played on a tragic note, as if foreshadowing Vesper's inevitable fate. That always felt wrong to me. When they would be on their romantic getaway in Venice, that music playing is always dreading, making me thing "yeah, this is obviously not going to end well". There's several moments in the film where Arnold could have played a different take on the material to suit the mood. Imagine a more laid back jazz orchestration for when Bond and Vesper are having dinner after having won the game, reflecting on how tensions between the two have been easing. A more whimsical take on it when they have a banter. Adding those layers would have gone a long way to giving that punch at the end when the music swells and becomes tragic (well, there had to be other factors for that to work beyond the music, but you get what I mean).
This is why I really like the reuse of it in QOS. In "What Keeps You Awake", Arnold plays it in an ambient way that's haunting, as if Vesper's ghost is watching over Bond's shoulder. For the ending at Russia it's more of a resolution taken care of but on a bittersweet note with those keyboards playing at the end. Arnold's reprising of "Vesper's Theme" in QOS at least proves that he could have given it more layers in CR but for whatever reason he never went for it the first time in 2006. Shame.