I left UK in August with Thailand visitor visa and spent 60 days in Thailand before flying to NZ for 6 weeks.

In NZ I obtained another visitor visa, flew to Thailand and stayed for about 6 weeks before flying to China for 8 days.

I returned to Thailand with no visa and got a 30 day entry.

After about 21 days I went to and stayed in Vientiane for 1 week and obtained a Thailand visitor

visa.

I returned to Thailand using this visa but stayed only for about 3 weeks before flying to Philippines.

Yesterday, I returned without visa and a 30 day entry.

My plans have changed and at the end of the month I want to return to Vientianne.

Then I want to come back to Thailand, stay 60 days, extend another 30 and then fly back to London.

Should I be able to apply for another vistor visa at the Thai embassy in Vientianne, is there any reason why they should refuse based on my travel history ?

You will be all right. There is no reason why they should deny you a visa, you could even ask for a double entry.

As said, no reason to decline.