You're missing out on a few things here. Firstly, Owen was DIRT CHEAP. £8 million? He'll pay that back in merchandising. He's not as big as Ronaldo or Beckham of course, but he's a very decent money-spinner in Asia.
Secondly, yes, Real have Raul and Ronaldo... but they don't actually have anyone else. They shipped Portillo off to Fiorentina. Morientes is still being offered up for loan, even though him playing in Europe seriously curtails any hopes they once had of offloading him. But that's it. Their strikeforce for the season would effectively be Raul and Ronaldo. No replacements, not even youth-teamers. We saw what happened to Real as they died down the stretch last year, and as brilliant as Ronaldo is he is injury-prone. What happens when Ronaldo is injured? Or tired? You need replacements. Since Owen was being offered up for peanuts, why not? You need striking cover and you got a very good one for something like three times less than Chelsea paid for DIDIER DROGBA.
(Side note: Real never "wanted" Eto'o. They OWN half of Eto'o already. Eto'o caused enough trouble for Valdano in the past that Real's only interest in him was to get a good transfer fee and ensure that he wouldn't be transferred to a rival like Barcelona, since he has made it his personal mission recently to destroy Real. They suceeded in the former and failed in the latter. There was never any question at any point that Eto'o would ever play for Real again.)
Thirdly, yes, Real needed Vieira. But this was never an either Owen or Vieira deal -- the Owen deal cost Real virtually nothing, which is why they wanted to pursue it alongside Vieira. The problem with the Vieira deal was that they were unwilling to pay him as much as Beckham, Zidane and Ronaldo. Owen accepted a lower wage than them, but Vieira refused. There is still a belief that Real will call Vieira's bluff before the Champions League deadline (August 31st) and offer him a fat new deal to tempt him.
And lastly, everyone is forgetting that Real DID actually sign a defender this year. They bought Walter Samuel from Roma very early in the off-season, remember? While Walter Samuel alone will not ensure that Real have a water-tight defence, they will be much better because he is VERY VERY good. In the current top 10 of defenders right now, definitely.
