Vietnam has sent 20,000 to 50,000 troops to the Chinese border area to strengthen its defenses, but analysts here say there is no sign yet of preparations for any major attack on China.

On the Chinese side of the border, railroad lines given over to transporting military equipment during China's invasion of Vietnam recently have returned to normal use, but a sizeable Chinese force remains along the border, the analysts said. China's official travel agency has failed so far to resume weekly tours of Nanning, a regional capital about 10 miles from the border.

Analysts here confirm reports of several active Vietnamese fortification projects, including what one Chinese complaint called "tens of thousands of Vietnamese army-men and civilians... building defense works in Land Son Province bordering China."

"I think the Vietnamese have been taught a lesson, that the Chinese are serious, that they will attack if provoked, so they want to be better prepared the next time," one analyst said. He did not rule out a Vietnamese attack on China, but said there were no signs of offensive preparations so far and suggested the Vietnamese might want to avoid inciting another massive Chinese invasion.

An official Vietnamese Foreign Ministry note to the Chinese Foreign Ministry, dated yesterday and released in full today, said talks on normalizing relations could begin March 29 if all Chinese troops were withdrawn from Vietnam by that time. If they did not withdraw, the note said, the Vietnamese would "use their legitimate self-defense right to defend the independence, sovereignty and territorial integrity of their fatherland."

Analysts said independent information tended to contradict the Vietnamese claim that Chinese troops remain "some places from 6 to 12 miles deep inside Vietnamese territory." But one analyst said, "I suspect quite strongly that the Chinese have held onto some border area hills" which the Vietnamese have used in the past to fire into Chinese territory.

Peking complained yesterday of Vietnamese "rifle and gunfire" into four separate Chinese border counties Sunday and Monday, "posing a serious threat to the life and property of Chinese inhabitants."

Analysts said the Vietnamese appeared to have moved about six divisions out of Cambodia, Laos and southern Vietnam in an apparent attempt to strengthen the border, although it was difficult to determine exactly how many had moved to the battle zone. Many troops had been observed moving toward the border on the Vietnamese side, while on the Chinese side there did not appear to be significant movement awary from the border, other than a few units being ceremonially welcomed back to provincial capitals in the area.

The Soviet Union has supplied the Vietnamese with more military equipment, including more Mig-21s and antiaircraft missiles, analysts said. They said it was difficult to judge the accuracy of the casualty figures used by both sides. Hanoi has reported more than 60,000 Chinese casualties, which analysis said is grossly inflated.

A foreign news agency report from Peking said Vice Premier. Deng Xiaoping (Teng Hsiao-Ping) privately put Vietnamese casualties at 37,000 dead, 7,000 wounded and 5,000 captured. Analysts say these casualty figures probably are inflated.

Repatriation of prisoners is expected to be a topic of discussion once the two sides meet, although the charges and countercharges hurled by Hanoi and Peking so far suggest talks will not take place for some time.