I hope you don't seriously believe this crap you just spewed out on here. Let's look at a few dates, shall we?
March 26, 2001 - The final episode of Nitro airs. It is announced that Vince McMahon purchased WCW after Fusient Media backed out of their deal with Eric Bishoff.
July 2001 - Booker T and Buff Bagwell have the first WCW match on WWF television. The following week, The Alliance forms, partnering the WCW wrestlers and ECW wrestlers as the Invasion storyline kicks into full gear.
November 2001 - Flair re-signs with WWF.
February 2002 - The NWO arrives in the WWF. This is eleven months after the purchase of WCW.
July 2002 - Eric Bischoff debuts as General Manager of RAW. This is sixteen months after the purchase of WCW.
October 2002 - Scott Steiner returns to WWE. This is nineteen months after the purchase of WCW.
March 31 2003 - Bill Goldberg debuts on RAW, two years after the purchase of WCW.
So, Flair was available eight months after the purchase of WCW, all of the other big names were available a year or more after the purchase. Which one of these guys should Vince have waited for? Flair was the only one that signed in 2001, and there was no guarantee that after the end of WCW he would even return to wrestling. The NWO wasn't available until almost a year after the sale of the company.
The fans that the WWF already had weren't going anywhere. Vince didn't need the WCW guys to build that fanbase, the core WWF fans could care less about most of the WCW guys. The fans he needed to attract were the core WCW fanbase, the ones that watched Nitro and didn't watch RAW. Waiting an entire year to start the angle would have resulted in many of those fans not even knowing it had happened. The hardcore WCW fan might watch for a few weeks to see if the WCW guys started showing up, or they might do what I did at the time and just follow along on websites like this one until they started seeing reports of the WCW guys appearing. The might have stuck around for a few weeks, maybe even a couple of months. But a year? Hardly.
Waiting that long to start the Invasion would have been stupid, plain and simple. Just from an economic standpoint, paying the talent they had already acquired to sit at home for months would have made no sense at all. Waiting for contracts to expire and hoping that those guys would sign once they did would also have made very little sense. It's easy to say in hindsight that of course they would have eventually signed, but are you going to pay twenty guys to do nothing on the chance that three or four guys will eventually sign with you, a year down the road? That's ridiculous.
One thing to keep in mind is that Vince didn't buy WCW to create the Invasion angle, pitting WWE against WCW wasn't the ultimate goal. He bought the company because he wanted the video library. That was far more valuable than all of the contracts combined. Getting some of the contracts along with the company name and library meant he also had an opportunity to run a company vs company angle. It was a bonus, not a goal, and it was one that had an expiration date. If he wasn't going to pull the trigger on it when he did, then he might as well have just repackaged everybody and brought them in under new gimmicks. After all, who is going to buy Booker T as the WCW champion if he hasn't defended the title in almost a year? That is what your suggestion would have resulted in...all of the titles that came over with the company would have sat undefended for at least eight months, if they waited til the Flair signing, or a year if they waited for the NWO.
As for the people saying he should have brought in Bischoff right away...seriously? Dumbest fucking thing I've ever read. Not because he wouldn't have added legitimacy to the group, but because at that time there was pretty much zero chance that it could have happened. A week before Vince bought WCW, Bischoff thought he had it in the bag. He appeared on Nitro to announce that the following week would be the Night of Champions. He knew that he was a few days away from owning WCW. The following Monday, Vince McMahon announced that he he had just purchased the company. Do you seriously think that, at that point in time, Eric Bischoff would have just said, "Aw shucks, I guess ya beat me. Sure, I'll come work for you." Of course not. The only thing that surprises me is that a year and a half later he finally did do that. But to have expected it to happen right after he got scooped on the purchase of WCW? No way.