Why did england become vulnerable to attack around the beginning of the fifth century

Because the previous military and political authority of the country: the Roman Empire, had left the island, and this created a power vacuum that could be capitalized by foreign invaders.

The foreign invaders that capitalized this power vacuum were the Angles, the Saxons, and the Jutes, all of them germanic peoples coming from modern-day Denmark. These groups invaded England and defeated the local Briton population which was confined to Wales, and Cornwall.