We have no evidence that Planet Vegeta has any space warning systems. We know that Star Trek Ships have planetary distance weaponry. Dropping out of warp, and launching attacks on the planet are probably not stoppable by anything we saw of the Saiyans at that time. Now later Super Saiyans probably would regard phasers, even ship based, as mild sunburns. However, as stated, can Saiyans get to the ships at planetary distances? They did get into orbital distances attacking Frieza but the ships can fire much farther away. We end up with Saiyans floating around in space for a bit.
Now, if on the other hand, Saiyans get to the Klingon planets, they are screwed. A single Saiyan might bust the planet if we take the original Vegeta and Nappa feat as canon.