I bought a old house three months ago.My toilet was super and flushing with gurgle.suddenly I have noticed

1-The level of water in bowl is about 1" (I have checked the flush tank and small tube eveything is ok)
2- first three flush go fast but fourth one go slow but no gurgle and 5 flush stay for few minutes
3-when taking bath in same bath ,see some bubles coming up from bowl
4- I suspect may be Vent pipe blocked ? Is it right? If so How I cleanit?through snake?how long?
For sure there is no toilet paper?

Your toilet was venting for some of the other fixtures because, as you suspect, the vent stack is clogged. I'd rod it from the roof with enough rod to reach roof to basement floor plus 20'. The toilet level drops because a clogged vent will allow siphoning to balance pressure in the drain lines.