Long-standing volunteers for Poppyscotland, known as 'members', gave their approval to the proposal at an extraordinary general meeting on Tuesday by 97 votes to two.
The proposed merger, which needs to be approved at a meeting of RBL trustees in June, would involve Poppyscotland retaining its current branding but replacing its board with a Scottish advisory committee that would include Ian McGregor, its chief executive. McGregor would also remain chief executive of Poppyscotland.
Poppyscotland would benefit from an additional £3m to invest in services over a three-year period if the merger went through, a spokeswoman said. The charity employs about 30 staff in offices in Edinburgh and Glasgow and has an annual income of about £3.2m.
If the merger goes ahead, Poppyscotland will continue to operate as a distinct charity within the RBL group. No redundancies would be made as a result of the merger, a spokeswoman for Poppyscotland said.
"A merger would enable Poppyscotland to provide more services for veterans in Scotland. It would also enable the Royal British Legion to operate in Scotland and we would be able to fundraise jointly."
The legion runs the annual poppy appeal in England, Wales and Northern Ireland.
In a statement, McGregor said: "Joining with our sister charity the Royal British Legion would mean a major boost for the ex-service community in Scotland. Veterans here have not been able to access the same optimal level of support as their counterparts elsewhere in the UK, and the merger will go a long way to addressing this."
A spokeswoman for the legion confirmed that a decision on the proposed merger would be made in June, but declined to comment further.