I want to know which verses or hymns of the Vedas declare Lord Brahma as supreme or highest?
Yajurveda Shatapatha Brahmana 184.108.40.206 declares Lord Brahma as Supreme. This is already discussed in How were the Trimurthi born?
You have answered this in your question itself. Lord Brahma is a very important figure in Vedic tradition. He is referred to as PrajApatI and Hiranyagarbha in the Vedas. This is also discussed here.
Also, if Lord Brahma is Prajapati then there are many verses or hymns dedicated to Prajapati and considered him as supreme as Indra, Agni, Surya (or Sun) and Vishnu.
When we can venerate Lord Brahma in the Vedic rituals then why not in general regular worship?
Due to two curses. However neither the below mentioned curses makes Lord Brahma inferior to other two Gods in Trinity. He still remains the God of Creation and has the equal importance as other two Gods in Trinity.
Only his worship is not popular or limited. Are there any eulogies and prayers to Brahma?
There is a temple of Lord Bhrahma in Pushkar, Rajasthan, I have visited this temple.
I. Lord Shiva cursed Lord Brahma
As per Shiva Purana
"You spoke untruth with a desire to become worshippable. It is my curse that, you will not be worshipped by anybody. You will posses only four heads."
Note: People only talk about curse, they should remember that in Shiva Purana, along with curse, Lord Shiva gave Brahma
a boon of being the presiding deity of all the yagya.
So in yagya, directly on indirectly Lord Brahma is worshiped.
II. Sage Bhrigu cursed Lord Brahma
As per Padma Purana
Sage Bhrigu became furious and cursed Lord Brahma--"Being intoxicated by your Rajoguna, you have shown disrespect to me. From today onwards people will not worship you."