How well Ichigo is written as a character has nothing to do with whether or not he could beat Vegeta in a fight. Sure, he may be a brick, but he's a brick that soul crushes Vegeta on sight passively.
As for your saying that Ichigo can't Reiatsu Crush, he has shown the capabilities of doing that (to himself), and has become far more experienced since then. Also, what you're failing to realize about how that whole mechanic works is that it doesn't really affect those who have strong Reiatsu themselves or strong resistances to Reiatsu. Hence, this why Ichigo can battle people like Aizen, Grimmjow, some of the Sternritter (who he held back against, so his Reiatsu wouldn't be as potent against them), etc. and none of them are insta-killed by his Reiatsu.
Secondly, let's talk about passive abilities and resistances. Reiatsu Crush is passive. Passive abilities work without having to think about them (or activating them in most cases). This means that Ichigo's Reiatsu Crush will be in effect the moment the battle starts. Now, resistances can provide the ability to resist the effects that certain hax have. Vegeta has no resistance to Soul Manipulation, and not the kind utilized in Reiatsu Crush, so Vegeta doesn't resist its effects, and thus would be killed by it.
And before you mention Hakai, it really doesn't matter. This is why I brought up passive abilities. Reiatsu Crush is passive and Hakai is not. The moment the battle begins, Vegeta will be crushed, and thus unable to perform Hakai. It's like it was stated earlier: