Dear Vu,
It's been a long time since we met last, and it's great to hear from you. I'm very glad that I could make it to your picnic party last week, but I'll come visit you soon.
I'm so happy that I can go on the picnic next week. Actually, I have some important business to attend and cannot leave home at that time. However, I can give up that and go on the picnic with you.
Thank you for inviting me, and please give my regards to your family members

Dear Vu,
It is with great pleasure that I accept your invitation to a picnic next week. It has been a long time since we both had some time off together and I am looking forward to it. I hope that you will be able to bring your family along with you. I’d like to make sure that they are comfortable too. I have already picked out a nice spot for us to have our picnic the park near my house offers plenty of room for children to run around and play. In addition, there is a small playground there as well so they can have fun while we talk and enjoy some food. If you have any questions or concerns about anything else, please let me know so we can work them out before our date!