It's rainy season in Phuket and Langkawi (doesn't rain all day but not the cloudless blue sky you'd expect). Rainiest months are September to November. Wouldn't recommend Bali or Phuket if you want a beach (pool sure). It's dry season in Bali.

Perhentians will be nice but not the easiest to get there. You'd have to fly to Kota Baru and take a 1 hour Grab (uber) ride down to Besut jetty that'd cost around $20USD one way. There's only 1 major international chain hotel there and that's Marriott. $200/night will not fit your budget at the Marriott Perhentian (more like $300+/night). The rest of the accommodation in Perhentian can range from a backpacker campsite to a nicer hotel. Don't recall any hotels with a nice pool at Perhentian other than the Marriott and maybe Mimpi resort. Most places in Perhentian serves the same Malaysian food with a few international options.

Tioman will be nice but you'd need to drive or take a bus there. Other places in Malaysia include Kota Kinabalu with nice international chain hotels and you can take day trips to nearby islands (or you can stay at some of the islands resorts too e.g. Gaya Island Resort / Bungaraya Resort or go to Pulau Mantanani [island]). Plenty of culture and local food in Kota Kinabalu. If you are more adventurous, you could drive to Kudat and stay near the tip of Borneo (Simpang Megayau) - the northern most point of Borneo island. There are a few nice locally run beach villas there.

Vietnam will be very hot in July. Koh Samui is very much overdeveloped from what I hear but I haven't been back in over 15 years. There are no direct flights from Koh Samui to KL tho.